diff --git a/src/sequence_diagram/visitor/translation_unit_visitor.cc b/src/sequence_diagram/visitor/translation_unit_visitor.cc
index a277888d..0a8e0ff1 100644
--- a/src/sequence_diagram/visitor/translation_unit_visitor.cc
+++ b/src/sequence_diagram/visitor/translation_unit_visitor.cc
@@ -467,9 +467,19 @@ bool translation_unit_visitor::VisitCallExpr(clang::CallExpr *expr)
                     .substr();
             m.message_name = message_name.substr(0, message_name.size() - 2);
         }
-        m.return_type =
-            function_call_expr->getCallReturnType(current_ast_context)
-                .getAsString();
+
+        const auto &return_type =
+            function_call_expr->getCallReturnType(current_ast_context);
+
+        //
+        // Without this if, this crashes test case t20003 on LLVM <= 12
+        //
+        if (!clang::dyn_cast_or_null<clang::UnresolvedLookupExpr>(
+                function_call_expr->getCallee())) {
+            m.return_type = return_type.getAsString();
+        }
+        else
+            m.return_type = "";
     }
     else {
         return true;