diff --git a/clang-tools-extra/clang-tidy/bugprone/NotNullTerminatedResultCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/NotNullTerminatedResultCheck.cpp
--- a/clang-tools-extra/clang-tidy/bugprone/NotNullTerminatedResultCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/NotNullTerminatedResultCheck.cpp
@@ -802,11 +802,14 @@
     while (It != PP->macro_end() && !AreSafeFunctionsWanted.hasValue()) {
       if (It->first->getName() == "__STDC_WANT_LIB_EXT1__") {
         const auto *MI = PP->getMacroInfo(It->first);
-        const auto &T = MI->tokens().back();
-        StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
-        llvm::APInt IntValue;
-        ValueStr.getAsInteger(10, IntValue);
-        AreSafeFunctionsWanted = IntValue.getZExtValue();
+        // PP->getMacroInfo() returns nullptr if macro has no definition.
+        if (MI) {
+          const auto &T = MI->tokens().back();
+          StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
+          llvm::APInt IntValue;
+          ValueStr.getAsInteger(10, IntValue);
+          AreSafeFunctionsWanted = IntValue.getZExtValue();
+        }
       }
 
       ++It;
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone-not-null-terminated-result-undef-stdc-want-lib-ext1.c b/clang-tools-extra/test/clang-tidy/checkers/bugprone-not-null-terminated-result-undef-stdc-want-lib-ext1.c
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone-not-null-terminated-result-undef-stdc-want-lib-ext1.c
@@ -0,0 +1,17 @@
+// RUN: %check_clang_tidy %s bugprone-not-null-terminated-result %t -- \
+// RUN: -- -std=c11 -I %S/Inputs/bugprone-not-null-terminated-result
+
+#include "not-null-terminated-result-c.h"
+
+#define __STDC_LIB_EXT1__ 1
+#define __STDC_WANT_LIB_EXT1__ 1
+#undef __STDC_WANT_LIB_EXT1__
+
+void f(const char *src) {
+  char dest[13];
+  memcpy_s(dest, 13, src, strlen(src) - 1);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: the result from calling 'memcpy_s' is not null-terminated [bugprone-not-null-terminated-result]
+  // CHECK-FIXES: char dest[14];
+  // CHECK-FIXES-NEXT: strncpy_s(dest, 14, src, strlen(src) - 1);
+}
+