diff --git a/simplecpp.cpp b/simplecpp.cpp
index d8880355..b8dd063d 100755
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -2125,7 +2125,7 @@ namespace simplecpp {
                         if (expandArg(&temp, defToken, parametertokens))
                             macroName = temp.cback()->str();
                         if (expandArg(&temp, defToken->next->next->next, parametertokens))
-                            macroName += temp.cback()->str();
+                            macroName += temp.cback() ? temp.cback()->str() : "";
                         else
                             macroName += defToken->next->next->next->str();
                         lastToken = defToken->next->next->next;
diff --git a/test.cpp b/test.cpp
index 3e245516..82b9e1c7 100644
--- a/test.cpp
+++ b/test.cpp
@@ -1717,6 +1717,17 @@ static void ifDefinedHashHash()
     ASSERT_EQUALS("file0,4,#error,#error FOO is enabled\n", toString(outputList));
 }
 
+static void ifDefinedHashHash2()
+{
+    // #409
+    // do not crash when expanding P()  (as ## rhs is "null")
+    // note: gcc outputs "defined E"
+    const char code[] = "#define P(p)defined E##p\n"
+                        "P()\n";
+    simplecpp::OutputList outputList;
+    ASSERT_EQUALS("\n0", preprocess(code, &outputList));
+}
+
 static void ifLogical()
 {
     const char code[] = "#if defined(A) || defined(B)\n"
@@ -3149,6 +3160,7 @@ int main(int argc, char **argv)
     TEST_CASE(ifDefinedInvalid1);
     TEST_CASE(ifDefinedInvalid2);
     TEST_CASE(ifDefinedHashHash);
+    TEST_CASE(ifDefinedHashHash2);
     TEST_CASE(ifLogical);
     TEST_CASE(ifSizeof);
     TEST_CASE(elif);