diff --git a/etc.c b/etc.c
index 7fabd03..0c3a004 100644
--- a/etc.c
+++ b/etc.c
@@ -2009,18 +2009,19 @@ static char Base64Table[] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 char *
-base64_encode(const unsigned char *src, int len)
+base64_encode(const unsigned char *src, size_t len)
 {
     unsigned char *w, *at;
     const unsigned char *in, *endw;
-    int j, k;
+    int j;
+    size_t k;
 
     k = len;
     if (k % 3)
       k += 3 - (k % 3);
     k = k / 3 * 4;
 
-    if (k < len)
+    if (k + 1 < len)
 	return NULL;
 
     w = GC_MALLOC_ATOMIC(k + 1);
diff --git a/proto.h b/proto.h
index 6aafbb4..899eec8 100644
--- a/proto.h
+++ b/proto.h
@@ -829,4 +829,4 @@ void srand48(long);
 long lrand48(void);
 #endif
 
-extern char *base64_encode(const unsigned char *src, int len);
+extern char *base64_encode(const unsigned char *src, size_t len);