diff --git a/src/sbbs3/chat.cpp b/src/sbbs3/chat.cpp
index 66494c4d7bc81c2ae4cd7d8938decc26b45b357f..9f900d77e8dc4c3cca3116ecb438000cd9a93c38 100644
--- a/src/sbbs3/chat.cpp
+++ b/src/sbbs3/chat.cpp
@@ -1552,7 +1552,7 @@ void sbbs_t::guruchat(char* line, char* gurubuf, int gurunum, char* last_answer)
 	j=strlen(line);
 	k=0;
 	for(i=0;i<j;i++) {
-		if(line[i]<0 || !isalnum(line[i])) {
+		if(line[i]<0 || !isalnum((uchar)line[i])) {
 			if(!k)	/* beginning non-alphanumeric */
 				continue;
 			if(line[i]==line[i+1])	/* redundant non-alnum */
@@ -1565,7 +1565,7 @@ void sbbs_t::guruchat(char* line, char* gurubuf, int gurunum, char* last_answer)
 	cstr[k]=0;
 	while(k) {
 		k--;
-		if(!isalnum(cstr[k]))
+		if(!isalnum((uchar)cstr[k]))
 			continue;
 		break; 
 	}
@@ -1754,8 +1754,8 @@ void sbbs_t::guruchat(char* line, char* gurubuf, int gurunum, char* last_answer)
 			if(action!=NODE_MCHT) {
 				for(i=0;i<k;i++) {
 					if(i && mistakes && theanswer[i]!=theanswer[i-1] &&
-						((!isalnum(theanswer[i]) && !sbbs_random(100))
-						|| (isalnum(theanswer[i]) && !sbbs_random(30)))) {
+						((!isalnum((uchar)theanswer[i]) && !sbbs_random(100))
+						|| (isalnum((uchar)theanswer[i]) && !sbbs_random(30)))) {
 						c=j=((uint)sbbs_random(3)+1);	/* 1 to 3 chars */
 						if(c<strcspn(theanswer+(i+1),"\0., "))
 							c=j=1;
@@ -1917,13 +1917,13 @@ bool sbbs_t::guruexp(char **ptrptr, char *line)
 		else {
 			cp=strstr(line,str);
 			if(cp && c) {
-				if(cp!=line || isalnum(*(cp+strlen(str))))
+				if(cp!=line || isalnum((uchar)*(cp+strlen(str))))
 					cp=0; 
 			}
 			else {	/* must be isolated word */
 				while(cp)
-					if((cp!=line && isalnum(*(cp-1)))
-						|| isalnum(*(cp+strlen(str))))
+					if((cp!=line && isalnum((uchar)*(cp-1)))
+						|| isalnum((uchar)*(cp+strlen(str))))
 						cp=strstr(cp+strlen(str),str);
 					else
 						break;