diff --git a/src/load_abc.cpp b/src/load_abc.cpp
index ee79f39..874ab8f 100644
--- a/src/load_abc.cpp
+++ b/src/load_abc.cpp
@@ -268,7 +268,8 @@ static void setenv(const char *name, const char *value, int overwrite)
 #endif
 
 static int abc_isvalidchar(char c) {
-	return(isalpha(c) || isdigit(c) || isspace(c) || c == '%' || c == ':');
+	unsigned char u = static_cast<unsigned char>(c);
+	return(isalpha(u) || isdigit(u) || isspace(u) || c == '%' || c == ':');
 }
 #if 0
 static const char *abc_skipspace(const char *p)