Hedge against the "zero Value"

Reflect in Go has the concept of a "zero Value" [1] (not be confused with a
type's zero value with a lowercase "v") and asking for Type on one will
panic. I'm not exactly sure under what conditions these are generated,
but they are occasionally, so here we hedge against them.

See also: #75

Fixes #75.

[1] https://golang.org/pkg/reflect/#Value

generator/datareplacer/datareplacer.go

@@ -46,5 +46,18 @@ func ReplaceData(requestData map[string]interface{}, responseData map[string]int
 }
 
 func isSameType(v1, v2 interface{}) bool {
-	return reflect.ValueOf(v1).Type() == reflect.ValueOf(v2).Type()
+	v1Value := reflect.ValueOf(v1)
+	v2Value := reflect.ValueOf(v2)
+
+	// Reflect in Go has the concept of a "zero Value" (not be confused with a
+	// type's zero value with a lowercase "v") and asking for Type on one will
+	// panic. I'm not exactly sure under what conditions these are generated,
+	// but they are occasionally, so here we hedge against them.
+	//
+	// https://github.com/stripe/stripe-mock/issues/75
+	if !v1Value.IsValid() || !v2Value.IsValid() {
+		return false
+	}
+
+	return v1Value.Type() == v2Value.Type()
 }