diff --git a/src/base64.c b/src/base64.c
index 5c4ab3a..a01f0f6 100644
--- a/src/base64.c
+++ b/src/base64.c
@@ -167,9 +167,12 @@
 			 */
 
 			/* xx000000 xx001111 xx111122 xx222222 */
-			out[convlen]   = ((t[0] << 2) + (t[1] >> 4));
-			out[convlen+1] = ((t[1] << 4) + (t[2] >> 2));
-			out[convlen+2] = ((t[2] << 6) + (t[3] >> 0));
+			if (convlen < olen)
+				out[convlen]   = ((t[0] << 2) + (t[1] >> 4));
+			if (convlen+1 < olen)
+				out[convlen+1] = ((t[1] << 4) + (t[2] >> 2));
+			if (convlen+2 < olen)
+				out[convlen+2] = ((t[2] << 6) + (t[3] >> 0));
 
 			convlen += 3-pad;
 
@@ -237,9 +240,12 @@
 			 */
 
 			/* xx000000 xx001111 xx111122 xx222222 */
-			out[convlen]   = ((t[0] << 2) + (t[1] >> 4));
-			out[convlen + 1] = ((t[1] << 4) + (t[2] >> 2));
-			out[convlen + 2] = ((t[2] << 6) + (t[3] >> 0));
+			if (convlen < olen)
+				out[convlen]   = ((t[0] << 2) + (t[1] >> 4));
+			if (convlen+1 < olen)
+				out[convlen+1] = ((t[1] << 4) + (t[2] >> 2));
+			if (convlen+2 < olen)
+				out[convlen+2] = ((t[2] << 6) + (t[3] >> 0));
 
 			convlen += 3;
 			i = 0;