diff --git a/user/base64.c b/user/base64.c
index 7714006..cc046ac 100644
--- a/user/base64.c
+++ b/user/base64.c
@@ -50,7 +50,7 @@ int ICACHE_FLASH_ATTR base64_decode(size_t in_len, const char *in, size_t out_le
 
 	for(io=0,ii=0,v=0,rem=0;ii<in_len;ii++) {
 		unsigned char ch;
-		if(isspace(in[ii])) continue;
+		if(isspace((int)in[ii])) continue;
 		if(in[ii]=='=') break; /* stop at = */
 		ch=base64dec_tab[(unsigned int)in[ii]];
 		if(ch==255) break; /* stop at a parse error */