diff --git a/src/dec/idec_dec.c b/src/dec/idec_dec.c
index 7c0bb4d1..cf8a33a4 100644
--- a/src/dec/idec_dec.c
+++ b/src/dec/idec_dec.c
@@ -141,8 +141,14 @@ static void DoRemap(WebPIDecoder* const idec, ptrdiff_t offset) {
       }
       {
         const uint8_t* const last_start = dec->parts[last_part].buf;
-        VP8BitReaderSetBuffer(&dec->parts[last_part], last_start,
-                              mem->buf + mem->end - last_start);
+        // 'last_start' will be NULL when 'idec->state' is < STATE_VP8_PARTS0
+        // and through a portion of that state (when there isn't enough data to
+        // parse the partitions). The bitreader is only used meaningfully when
+        // there is enough data to begin parsing partition 0.
+        if (last_start != NULL) {
+          VP8BitReaderSetBuffer(&dec->parts[last_part], last_start,
+                                mem->buf + mem->end - last_start);
+        }
       }
       if (NeedCompressedAlpha(idec)) {
         ALPHDecoder* const alph_dec = dec->alph_dec;
diff --git a/src/utils/bit_reader_utils.c b/src/utils/bit_reader_utils.c
index ec9a2341..5e3a8b37 100644
--- a/src/utils/bit_reader_utils.c
+++ b/src/utils/bit_reader_utils.c
@@ -31,12 +31,11 @@
 void VP8BitReaderSetBuffer(VP8BitReader* const br,
                            const uint8_t* const start,
                            size_t size) {
-  if (start != NULL) {
-    br->buf = start;
-    br->buf_end = start + size;
-    br->buf_max =
-        (size >= sizeof(lbit_t)) ? start + size - sizeof(lbit_t) + 1 : start;
-  }
+  assert(start != NULL);
+  br->buf = start;
+  br->buf_end = start + size;
+  br->buf_max =
+      (size >= sizeof(lbit_t)) ? start + size - sizeof(lbit_t) + 1 : start;
 }
 
 void VP8InitBitReader(VP8BitReader* const br,