diff --git a/clang/lib/AST/ASTImporterLookupTable.cpp b/clang/lib/AST/ASTImporterLookupTable.cpp
index bf772c20d32ee..ef42561c6f941 100644
--- a/clang/lib/AST/ASTImporterLookupTable.cpp
+++ b/clang/lib/AST/ASTImporterLookupTable.cpp
@@ -14,6 +14,7 @@
 #include "clang/AST/ASTImporterLookupTable.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/RecursiveASTVisitor.h"
+#include "llvm/Support/FormatVariadic.h"
 
 namespace clang {
 
@@ -93,10 +94,19 @@ void ASTImporterLookupTable::add(DeclContext *DC, NamedDecl *ND) {
 }
 
 void ASTImporterLookupTable::remove(DeclContext *DC, NamedDecl *ND) {
-  DeclList &Decls = LookupTable[DC][ND->getDeclName()];
+  const DeclarationName Name = ND->getDeclName();
+  DeclList &Decls = LookupTable[DC][Name];
   bool EraseResult = Decls.remove(ND);
   (void)EraseResult;
-  assert(EraseResult == true && "Trying to remove not contained Decl");
+#ifndef NDEBUG
+  if (!EraseResult) {
+    std::string Message =
+        llvm::formatv("Trying to remove not contained Decl '{0}' of type {1}",
+                      Name.getAsString(), DC->getDeclKindName())
+            .str();
+    llvm_unreachable(Message.c_str());
+  }
+#endif
 }
 
 void ASTImporterLookupTable::add(NamedDecl *ND) {