diff --git a/lib/checktype.cpp b/lib/checktype.cpp
index 822d961a6..3b87f51c1 100644
--- a/lib/checktype.cpp
+++ b/lib/checktype.cpp
@@ -248,10 +248,16 @@ void CheckType::checkSignConversion()
                     continue; // Todo: properly handle casts, function calls, etc
                 const Variable *var = tok1->variable();
                 if (var && tok1->getValueLE(-1,_settings)) {
-                    bool signedvar = false;
+                    bool signedvar = true;  // assume that variable is signed since it can have a negative value
                     for (const Token *type = var->typeStartToken();; type = type->next()) {
-                        if (type->isSigned()) {
-                            signedvar = true;
+                        if (type->isUnsigned()) {
+                            signedvar = false;
+                            break;
+                        }
+                        if (type->isSigned())
+                            break;
+                        if (type->isName() && !Token::Match(type, "char|short|int|long|const")) {
+                            signedvar = false;
                             break;
                         }
                         if (type == var->typeEndToken())
diff --git a/test/testtype.cpp b/test/testtype.cpp
index d9e0dd66b..9378a14c7 100644
--- a/test/testtype.cpp
+++ b/test/testtype.cpp
@@ -105,11 +105,17 @@ private:
     }
 
     void signConversion() {
-        check("unsigned int f1(signed int x, unsigned int y) {"
+        check("unsigned int f1(signed int x, unsigned int y) {" // x is signed
               "  return x * y;\n"
               "}\n"
               "void f2() { f1(-4,4); }");
         ASSERT_EQUALS("[test.cpp:1]: (warning) Suspicious code: sign conversion of x in calculation, even though x can have a negative value\n", errout.str());
+
+        check("unsigned int f1(int x) {" // x has no signedness, but it can have the value -1 so assume it's signed
+              "  return x * 5U;\n"
+              "}\n"
+              "void f2() { f1(-4); }");
+        ASSERT_EQUALS("[test.cpp:1]: (warning) Suspicious code: sign conversion of x in calculation, even though x can have a negative value\n", errout.str());
     }
 };