diff --git a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
--- a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
+++ b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
@@ -534,6 +534,11 @@
   // macro definition for it.
   void MacroUndefined(const Token &MacroNameToken, const MacroDefinition &MD,
                       const MacroDirective *Undef) override {
+    // If this macro wasn't previously defined we don't need to do anything
+    // here.
+    if (!Undef)
+      return;
+
     llvm::erase_if(PendingMacros, [&MD](const PendingMacro &PM) {
       return MD.getMacroInfo()->getDefinitionLoc() ==
              PM.MD->getMacroInfo()->getDefinitionLoc();
diff --git a/clang/test/ExtractAPI/macro_undefined.c b/clang/test/ExtractAPI/macro_undefined.c
--- a/clang/test/ExtractAPI/macro_undefined.c
+++ b/clang/test/ExtractAPI/macro_undefined.c
@@ -19,6 +19,8 @@
 FUNC_GEN(foo)
 FUNC_GEN(bar, const int *, unsigned);
 #undef FUNC_GEN
+// Undefining a not previously defined macro should not result in a crash.
+#undef FOO
 
 //--- reference.output.json.in
 {