diff --git a/source/compiler/sc.h b/source/compiler/sc.h
index 249c6f3..4ae1231 100644
--- a/source/compiler/sc.h
+++ b/source/compiler/sc.h
@@ -549,8 +549,8 @@ SC_FUNC int tokeninfo(cell *val,char **str);
 SC_FUNC int needtoken(int token);
 SC_FUNC void litadd(cell value);
 SC_FUNC void litinsert(cell value,int pos);
-SC_FUNC int alphanum(char c);
-SC_FUNC int ishex(char c);
+SC_FUNC int alphanum(unsigned char c);
+SC_FUNC int ishex(unsigned char c);
 SC_FUNC void delete_symbol(symbol *root,symbol *sym);
 SC_FUNC void delete_symbols(symbol *root,int level,int del_labels,int delete_functions);
 SC_FUNC int refer_symbol(symbol *entry,symbol *bywhom);
diff --git a/source/compiler/sc2.c b/source/compiler/sc2.c
index 14d62de..157f17a 100644
--- a/source/compiler/sc2.c
+++ b/source/compiler/sc2.c
@@ -45,7 +45,7 @@ static symbol *find_symbol(const symbol *root,const char *name,int fnumber,int a
 
 static void substallpatterns(unsigned char *line,int buffersize);
 static int match(char *st,int end);
-static int alpha(char c);
+static int alpha(unsigned char c);
 
 #define SKIPMODE      1 /* bit field in "#if" stack */
 #define PARSEMODE     2 /* bit field in "#if" stack */
@@ -2502,7 +2502,7 @@ static cell litchar(const unsigned char **lptr,int flags)
  *  Test if character "c" is alphabetic ("a".."z"), an underscore ("_")
  *  or an "at" sign ("@"). The "@" is an extension to standard C.
  */
-static int alpha(char c)
+static int alpha(unsigned char c)
 {
   return (isalpha(c) || c=='_' || c==PUBLIC_CHAR);
 }
@@ -2511,7 +2511,7 @@ static int alpha(char c)
  *
  *  Test if character "c" is alphanumeric ("a".."z", "0".."9", "_" or "@")
  */
-SC_FUNC int alphanum(char c)
+SC_FUNC int alphanum(unsigned char c)
 {
   return (alpha(c) || isdigit(c));
 }
@@ -2520,7 +2520,7 @@ SC_FUNC int alphanum(char c)
  *
  *  Test if character "c" is a hexadecimal digit ("0".."9" or "a".."f").
  */
-SC_FUNC int ishex(char c)
+SC_FUNC int ishex(unsigned char c)
 {
   return (c>='0' && c<='9') || (c>='a' && c<='f') || (c>='A' && c<='F');
 }