diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index 9194186e1..a273749e4 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -1646,6 +1646,8 @@ static void valueFlowFunctionDefaultParameter(TokenList *tokenlist, SymbolDataba
             const Variable* var = function->getArgumentVar(arg);
             if (var && var->hasDefault() && Token::Match(var->nameToken(), "%var% = %num%|%str% [,)]")) {
                 const Token* valueTok = var->nameToken()->tokAt(2);
+                if (valueTok->values.empty())
+                    continue;
                 const_cast<Token*>(valueTok)->values.front().defaultArg = true;
                 valueFlowInjectParameter(tokenlist, errorLogger, settings, var, scope, valueTok->values);
             }
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index fbfd968bd..804fb0add 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -62,6 +62,8 @@ private:
         TEST_CASE(valueFlowForLoop);
         TEST_CASE(valueFlowSubFunction);
         TEST_CASE(valueFlowFunctionReturn);
+
+        TEST_CASE(valueFlowFunctionDefaultParameter);
     }
 
     bool testValueOfX(const char code[], unsigned int linenr, int value) {
@@ -1422,6 +1424,15 @@ private:
                "}";
         ASSERT_EQUALS(15, valueOfTok(code, "*").intvalue);
     }
+
+    void valueFlowFunctionDefaultParameter() {
+        const char *code;
+
+        code = "class continuous_src_time {\n"
+               "    continuous_src_time(std::complex<double> f, double st = 0.0, double et = infinity) {}\n"
+               "};";
+        testValueOfX(code, 2U, 2); // Don't crash (#6494)
+    }
 };
 
 REGISTER_TEST(TestValueFlow)