diff --git a/flang-rt/include/flang-rt/runtime/io-stmt.h b/flang-rt/include/flang-rt/runtime/io-stmt.h
index 0b3194da0ab4..9f71d515cb61 100644
--- a/flang-rt/include/flang-rt/runtime/io-stmt.h
+++ b/flang-rt/include/flang-rt/runtime/io-stmt.h
@@ -438,7 +438,9 @@ template <>
 class ListDirectedStatementState<Direction::Input>
     : public FormattedIoStatementState<Direction::Input> {
 public:
-  RT_API_ATTRS bool inNamelistSequence() const { return inNamelistSequence_; }
+  RT_API_ATTRS const NamelistGroup *namelistGroup() const {
+    return namelistGroup_;
+  }
   RT_API_ATTRS int EndIoStatement();
 
   // Skips value separators, handles repetition and null values.
@@ -451,18 +453,19 @@ public:
   // input statement.  This member function resets some state so that
   // repetition and null values work correctly for each successive
   // NAMELIST input item.
-  RT_API_ATTRS void ResetForNextNamelistItem(bool inNamelistSequence) {
+  RT_API_ATTRS void ResetForNextNamelistItem(
+      const NamelistGroup *namelistGroup) {
     remaining_ = 0;
     if (repeatPosition_) {
       repeatPosition_->Cancel();
     }
     eatComma_ = false;
     realPart_ = imaginaryPart_ = false;
-    inNamelistSequence_ = inNamelistSequence;
+    namelistGroup_ = namelistGroup;
   }
 
 protected:
-  bool inNamelistSequence_{false};
+  const NamelistGroup *namelistGroup_{nullptr};
 
 private:
   int remaining_{0}; // for "r*" repetition
diff --git a/flang-rt/lib/runtime/edit-input.cpp b/flang-rt/lib/runtime/edit-input.cpp
index 80cc085d9954..4f01623c6cf1 100644
--- a/flang-rt/lib/runtime/edit-input.cpp
+++ b/flang-rt/lib/runtime/edit-input.cpp
@@ -534,7 +534,7 @@ static RT_API_ATTRS ScannedRealInput ScanRealInput(
       next = io.NextInField(remaining, edit);
     }
     if (!next || *next == ')') { // NextInField fails on separators like ')'
-      std::size_t byteCount{0};
+      std::size_t byteCount{1};
       if (!next) {
         next = io.GetCurrentChar(byteCount);
       }
diff --git a/flang-rt/lib/runtime/io-stmt.cpp b/flang-rt/lib/runtime/io-stmt.cpp
index af44a9d6f272..e08088fab431 100644
--- a/flang-rt/lib/runtime/io-stmt.cpp
+++ b/flang-rt/lib/runtime/io-stmt.cpp
@@ -1086,7 +1086,7 @@ ChildListIoStatementState<DIR>::ChildListIoStatementState(
   if constexpr (DIR == Direction::Input) {
     if (auto *listInput{child.parent()
                 .get_if<ListDirectedStatementState<Direction::Input>>()}) {
-      this->inNamelistSequence_ = listInput->inNamelistSequence();
+      this->namelistGroup_ = listInput->namelistGroup();
     }
   }
 #else
diff --git a/flang-rt/lib/runtime/namelist.cpp b/flang-rt/lib/runtime/namelist.cpp
index cbc32262ffc2..44a8fe2de3cc 100644
--- a/flang-rt/lib/runtime/namelist.cpp
+++ b/flang-rt/lib/runtime/namelist.cpp
@@ -44,8 +44,7 @@ bool IODEF(OutputNamelist)(Cookie cookie, const NamelistGroup &group) {
     if ((connection.NeedAdvance(prefixLen) &&
             !(io.AdvanceRecord() && EmitAscii(io, " ", 1))) ||
         !EmitAscii(io, prefix, prefixLen) ||
-        (connection.NeedAdvance(
-             Fortran::runtime::strlen(str) + (suffix != ' ')) &&
+        (connection.NeedAdvance(runtime::strlen(str) + (suffix != ' ')) &&
             !(io.AdvanceRecord() && EmitAscii(io, " ", 1)))) {
       return false;
     }
@@ -102,8 +101,8 @@ static constexpr RT_API_ATTRS char NormalizeIdChar(char32_t ch) {
   return static_cast<char>(ch >= 'A' && ch <= 'Z' ? ch - 'A' + 'a' : ch);
 }
 
-static RT_API_ATTRS bool GetLowerCaseName(
-    IoStatementState &io, char buffer[], std::size_t maxLength) {
+static RT_API_ATTRS bool GetLowerCaseName(IoStatementState &io, char buffer[],
+    std::size_t maxLength, bool crashIfTooLong = true) {
   std::size_t byteLength{0};
   if (auto ch{io.GetNextNonBlank(byteLength)}) {
     if (IsLegalIdStart(*ch)) {
@@ -117,8 +116,10 @@ static RT_API_ATTRS bool GetLowerCaseName(
       if (j <= maxLength) {
         return true;
       }
-      io.GetIoErrorHandler().SignalError(
-          "Identifier '%s...' in NAMELIST input group is too long", buffer);
+      if (crashIfTooLong) {
+        io.GetIoErrorHandler().SignalError(
+            "Identifier '%s...' in NAMELIST input group is too long", buffer);
+      }
     }
   }
   return false;
@@ -356,9 +357,8 @@ static RT_API_ATTRS bool HandleComponent(IoStatementState &io, Descriptor &desc,
     const DescriptorAddendum *addendum{source.Addendum()};
     if (const typeInfo::DerivedType *
         type{addendum ? addendum->derivedType() : nullptr}) {
-      if (const typeInfo::Component *
-          comp{type->FindDataComponent(
-              compName, Fortran::runtime::strlen(compName))}) {
+      if (const typeInfo::Component *comp{
+              type->FindDataComponent(compName, runtime::strlen(compName))}) {
         bool createdDesc{false};
         if (comp->rank() > 0 && source.rank() > 0) {
           // If base and component are both arrays, the component name
@@ -484,7 +484,7 @@ bool IODEF(InputNamelist)(Cookie cookie, const NamelistGroup &group) {
       handler.SignalError("NAMELIST input group has no name");
       return false;
     }
-    if (Fortran::runtime::strcmp(group.groupName, name) == 0) {
+    if (runtime::strcmp(group.groupName, name) == 0) {
       break; // found it
     }
     SkipNamelistGroup(io);
@@ -503,7 +503,7 @@ bool IODEF(InputNamelist)(Cookie cookie, const NamelistGroup &group) {
     }
     std::size_t itemIndex{0};
     for (; itemIndex < group.items; ++itemIndex) {
-      if (Fortran::runtime::strcmp(name, group.item[itemIndex].name) == 0) {
+      if (runtime::strcmp(name, group.item[itemIndex].name) == 0) {
         break;
       }
     }
@@ -577,13 +577,14 @@ bool IODEF(InputNamelist)(Cookie cookie, const NamelistGroup &group) {
     if (const auto *addendum{useDescriptor->Addendum()};
         addendum && addendum->derivedType()) {
       const NonTbpDefinedIoTable *table{group.nonTbpDefinedIo};
-      listInput->ResetForNextNamelistItem(/*inNamelistSequence=*/true);
+      listInput->ResetForNextNamelistItem(&group);
       if (!IONAME(InputDerivedType)(cookie, *useDescriptor, table) &&
           handler.InError()) {
         return false;
       }
     } else {
-      listInput->ResetForNextNamelistItem(useDescriptor->rank() > 0);
+      listInput->ResetForNextNamelistItem(
+          useDescriptor->rank() > 0 ? &group : nullptr);
       if (!descr::DescriptorIO<Direction::Input>(io, *useDescriptor) &&
           handler.InError()) {
         return false;
@@ -607,27 +608,51 @@ bool IODEF(InputNamelist)(Cookie cookie, const NamelistGroup &group) {
 }
 
 RT_API_ATTRS bool IsNamelistNameOrSlash(IoStatementState &io) {
-  if (auto *listInput{
-          io.get_if<ListDirectedStatementState<Direction::Input>>()}) {
-    if (listInput->inNamelistSequence()) {
-      SavedPosition savedPosition{io};
-      std::size_t byteCount{0};
-      if (auto ch{io.GetNextNonBlank(byteCount)}) {
-        if (IsLegalIdStart(*ch)) {
-          do {
-            io.HandleRelativePosition(byteCount);
-            ch = io.GetCurrentChar(byteCount);
-          } while (ch && IsLegalIdChar(*ch));
-          ch = io.GetNextNonBlank(byteCount);
-          // TODO: how to deal with NaN(...) ambiguity?
-          return ch && (*ch == '=' || *ch == '(' || *ch == '%');
-        } else {
-          return *ch == '/' || *ch == '&' || *ch == '$';
-        }
-      }
+  auto *listInput{io.get_if<ListDirectedStatementState<Direction::Input>>()};
+  if (!listInput || !listInput->namelistGroup()) {
+    return false; // not namelist
+  }
+  SavedPosition savedPosition{io};
+  std::size_t byteCount{0};
+  auto ch{io.GetNextNonBlank(byteCount)};
+  if (!ch) {
+    return false;
+  } else if (!IsLegalIdStart(*ch)) {
+    return *ch == '/' || *ch == '&' || *ch == '$';
+  }
+  char id[nameBufferSize];
+  if (!GetLowerCaseName(io, id, sizeof id, /*crashIfTooLong=*/false)) {
+    return true; // long name
+  }
+  // It looks like a name, but might be "inf" or "nan".  Check what
+  // follows.
+  ch = io.GetNextNonBlank(byteCount);
+  if (!ch) {
+    return false;
+  } else if (*ch == '=' || *ch == '%') {
+    return true;
+  } else if (*ch != '(') {
+    return false;
+  } else if (runtime::strcmp(id, "nan") != 0) {
+    return true;
+  }
+  // "nan(" ambiguity
+  int depth{1};
+  while (true) {
+    io.HandleRelativePosition(byteCount);
+    ch = io.GetNextNonBlank(byteCount);
+    if (depth == 0) {
+      // nan(...) followed by '=', '%', or '('?
+      break;
+    } else if (!ch) {
+      return true; // not a valid NaN(...)
+    } else if (*ch == '(') {
+      ++depth;
+    } else if (*ch == ')') {
+      --depth;
     }
   }
-  return false;
+  return ch && (*ch == '=' || *ch == '%' || *ch == '(');
 }
 
 RT_OFFLOAD_API_GROUP_END
diff --git a/flang-rt/unittests/Runtime/Namelist.cpp b/flang-rt/unittests/Runtime/Namelist.cpp
index ee4018e491c3..f190bea14acf 100644
--- a/flang-rt/unittests/Runtime/Namelist.cpp
+++ b/flang-rt/unittests/Runtime/Namelist.cpp
@@ -334,4 +334,35 @@ TEST(NamelistTests, RealValueForInt) {
   EXPECT_EQ(got, expect);
 }
 
+TEST(NamelistTests, NanInputAmbiguity) {
+  OwningPtr<Descriptor> xDesc{// real :: x(5) = 0.
+      MakeArray<TypeCategory::Real, static_cast<int>(sizeof(float))>(
+          std::vector<int>{5}, std::vector<float>{{0, 0, 0, 0, 0}})};
+  OwningPtr<Descriptor> nanDesc{// real :: nan(2) = 0.
+      MakeArray<TypeCategory::Real, static_cast<int>(sizeof(float))>(
+          std::vector<int>{2}, std::vector<float>{{0, 0}})};
+  const NamelistGroup::Item items[]{{"x", *xDesc}, {"nan", *nanDesc}};
+  const NamelistGroup group{"nml", 2, items};
+  static char t1[]{"&nml x=1 2 nan(q) 4 nan(1)=5 nan(q)/"};
+  StaticDescriptor<1, true> statDesc;
+  Descriptor &internalDesc{statDesc.descriptor()};
+  internalDesc.Establish(TypeCode{CFI_type_char},
+      /*elementBytes=*/std::strlen(t1), t1, 0, nullptr, CFI_attribute_pointer);
+  auto inCookie{IONAME(BeginInternalArrayListInput)(
+      internalDesc, nullptr, 0, __FILE__, __LINE__)};
+  ASSERT_TRUE(IONAME(InputNamelist)(inCookie, group));
+  ASSERT_EQ(IONAME(EndIoStatement)(inCookie), IostatOk)
+      << "namelist real input for nans";
+  char out[40];
+  internalDesc.Establish(TypeCode{CFI_type_char}, /*elementBytes=*/sizeof out,
+      out, 0, nullptr, CFI_attribute_pointer);
+  auto outCookie{IONAME(BeginInternalArrayListOutput)(
+      internalDesc, nullptr, 0, __FILE__, __LINE__)};
+  ASSERT_TRUE(IONAME(OutputNamelist)(outCookie, group));
+  ASSERT_EQ(IONAME(EndIoStatement)(outCookie), IostatOk) << "namelist output";
+  std::string got{out, sizeof out};
+  static const std::string expect{" &NML X= 1. 2. NaN 4. 0.,NAN= 5. NaN/   "};
+  EXPECT_EQ(got, expect);
+}
+
 // TODO: Internal NAMELIST error tests