diff --git a/libavutil/base64.c b/libavutil/base64.c
index 25ae8c411c..2c1c94fc37 100644
--- a/libavutil/base64.c
+++ b/libavutil/base64.c
@@ -120,10 +120,12 @@ int av_base64_decode(uint8_t *out, const char *in_str, int out_size)
     }
 
 out3:
-    *dst++ = v >> 10;
+    if (end - dst)
+        *dst++ = v >> 10;
     v <<= 2;
 out2:
-    *dst++ = v >> 4;
+    if (end - dst)
+        *dst++ = v >> 4;
 out1:
 out0:
     return bits & 1 ? AVERROR_INVALIDDATA : dst - out;