diff --git a/clang/include/clang/Basic/OpenCLOptions.h b/clang/include/clang/Basic/OpenCLOptions.h
--- a/clang/include/clang/Basic/OpenCLOptions.h
+++ b/clang/include/clang/Basic/OpenCLOptions.h
@@ -37,12 +37,14 @@
   }
 
   bool isEnabled(llvm::StringRef Ext) const {
+    assert(OptMap.find(Ext) != OptMap.end() && "Unknown extension.");
     return OptMap.find(Ext)->second.Enabled;
   }
 
   // Is supported as either an extension or an (optional) core feature for
   // OpenCL version \p CLVer.
   bool isSupported(llvm::StringRef Ext, const LangOptions &LO) const {
+    assert(OptMap.find(Ext) != OptMap.end() && "Unknown extension.");
     // In C++ mode all extensions should work at least as in v2.0.
     auto CLVer = LO.OpenCLCPlusPlus ? 200 : LO.OpenCLVersion;
     auto I = OptMap.find(Ext)->getValue();
@@ -52,6 +54,7 @@
   // Is supported (optional) OpenCL core features for OpenCL version \p CLVer.
   // For supported extension, return false.
   bool isSupportedCore(llvm::StringRef Ext, const LangOptions &LO) const {
+    assert(OptMap.find(Ext) != OptMap.end() && "Unknown extension.");
     // In C++ mode all extensions should work at least as in v2.0.
     auto CLVer = LO.OpenCLCPlusPlus ? 200 : LO.OpenCLVersion;
     auto I = OptMap.find(Ext)->getValue();
@@ -61,6 +64,7 @@
   // Is supported OpenCL extension for OpenCL version \p CLVer.
   // For supported (optional) core feature, return false.
   bool isSupportedExtension(llvm::StringRef Ext, const LangOptions &LO) const {
+    assert(OptMap.find(Ext) != OptMap.end() && "Unknown extension.");
     // In C++ mode all extensions should work at least as in v2.0.
     auto CLVer = LO.OpenCLCPlusPlus ? 200 : LO.OpenCLVersion;
     auto I = OptMap.find(Ext)->getValue();