diff --git a/image/Decoder.cpp b/image/Decoder.cpp
index cbe7ce9fb214..9f177998b467 100644
--- a/image/Decoder.cpp
+++ b/image/Decoder.cpp
@@ -190,6 +190,7 @@ void
 Decoder::CompleteDecode()
 {
   // Implementation-specific finalization
+  BeforeFinishInternal();
   if (!HasError()) {
     FinishInternal();
   } else {
@@ -393,6 +394,7 @@ Decoder::AllocateFrameInternal(uint32_t aFrameNum,
  */
 
 void Decoder::InitInternal() { }
+void Decoder::BeforeFinishInternal() { }
 void Decoder::FinishInternal() { }
 void Decoder::FinishWithErrorInternal() { }
 
diff --git a/image/Decoder.h b/image/Decoder.h
index a88ff846ed84..f5295f6ecc0a 100644
--- a/image/Decoder.h
+++ b/image/Decoder.h
@@ -283,9 +283,14 @@ protected:
   /*
    * Internal hooks. Decoder implementations may override these and
    * only these methods.
+   *
+   * BeforeFinishInternal() can be used to detect if decoding is in an
+   * incomplete state, e.g. due to file truncation, in which case it should
+   * call PostDataError().
    */
   virtual void InitInternal();
   virtual void WriteInternal(const char* aBuffer, uint32_t aCount) = 0;
+  virtual void BeforeFinishInternal();
   virtual void FinishInternal();
   virtual void FinishWithErrorInternal();
 
diff --git a/image/decoders/nsBMPDecoder.cpp b/image/decoders/nsBMPDecoder.cpp
index 0c6aac9fa68c..97ad31265e1d 100644
--- a/image/decoders/nsBMPDecoder.cpp
+++ b/image/decoders/nsBMPDecoder.cpp
@@ -220,6 +220,14 @@ nsBMPDecoder::GetCompressedImageSize() const
        : mH.mImageSize;
 }
 
+void
+nsBMPDecoder::BeforeFinishInternal()
+{
+  if (!IsMetadataDecode() && !mImageData) {
+    PostDataError();
+  }
+}
+
 void
 nsBMPDecoder::FinishInternal()
 {
@@ -232,17 +240,18 @@ nsBMPDecoder::FinishInternal()
   // Send notifications if appropriate.
   if (!IsMetadataDecode() && HasSize()) {
 
+    // We should have image data.
+    MOZ_ASSERT(mImageData);
+
     // If it was truncated, fill in the missing pixels as black.
-    if (mImageData) {
-      while (mCurrentRow > 0) {
-        uint32_t* dst = RowBuffer();
-        while (mCurrentPos < mH.mWidth) {
-          SetPixel(dst, 0, 0, 0);
-          mCurrentPos++;
-        }
-        mCurrentPos = 0;
-        FinishRow();
+    while (mCurrentRow > 0) {
+      uint32_t* dst = RowBuffer();
+      while (mCurrentPos < mH.mWidth) {
+        SetPixel(dst, 0, 0, 0);
+        mCurrentPos++;
       }
+      mCurrentPos = 0;
+      FinishRow();
     }
 
     // Invalidate.
@@ -492,7 +501,7 @@ nsBMPDecoder::ReadInfoHeaderSize(const char* aData, size_t aLength)
     PostDataError();
     return Transition::TerminateFailure();
   }
-  // ICO BMPs must have a WinVMPv3 header. nsICODecoder should have already
+  // ICO BMPs must have a WinBMPv3 header. nsICODecoder should have already
   // terminated decoding if this isn't the case.
   MOZ_ASSERT_IF(mIsWithinICO, mH.mBIHSize == InfoHeaderLength::WIN_V3);
 
diff --git a/image/decoders/nsBMPDecoder.h b/image/decoders/nsBMPDecoder.h
index 28ff43f1c90e..f5773656f655 100644
--- a/image/decoders/nsBMPDecoder.h
+++ b/image/decoders/nsBMPDecoder.h
@@ -151,6 +151,7 @@ public:
 
   virtual void WriteInternal(const char* aBuffer,
                              uint32_t aCount) override;
+  virtual void BeforeFinishInternal() override;
   virtual void FinishInternal() override;
 
 private: