diff --git a/netwerk/test/gtest/moz.build b/netwerk/test/gtest/moz.build
index 6e6c80152..e463feb65 100644
--- a/netwerk/test/gtest/moz.build
+++ b/netwerk/test/gtest/moz.build
@@ -5,6 +5,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 UNIFIED_SOURCES += [
+    'TestBase64Stream.cpp',
     'TestProtocolProxyService.cpp',
     'TestStandardURL.cpp',
 ]
diff --git a/xpcom/io/Base64.cpp b/xpcom/io/Base64.cpp
index 911c0595a..b9fa7baf8 100644
--- a/xpcom/io/Base64.cpp
+++ b/xpcom/io/Base64.cpp
@@ -108,30 +108,51 @@ EncodeInputStream_Encoder(nsIInputStream* aStream,
   EncodeInputStream_State<T>* state =
     static_cast<EncodeInputStream_State<T>*>(aClosure);
 
+  // We always consume all data.
+  *aWriteCount = aCount;
+
   // If we have any data left from last time, encode it now.
   uint32_t countRemaining = aCount;
   const unsigned char* src = (const unsigned char*)aFromSegment;
   if (state->charsOnStack) {
+    MOZ_ASSERT(state->charsOnStack == 1 || state->charsOnStack == 2);
+
+    // Not enough data to compose a triple.
+    if (state->charsOnStack == 1 && countRemaining == 1) {
+      state->charsOnStack = 2;
+      state->c[1] = src[0];
+      return NS_OK;
+    }
+
+    uint32_t consumed = 0;
     unsigned char firstSet[4];
     if (state->charsOnStack == 1) {
       firstSet[0] = state->c[0];
       firstSet[1] = src[0];
-      firstSet[2] = (countRemaining > 1) ? src[1] : '\0';
+      firstSet[2] = src[1];
       firstSet[3] = '\0';
+      consumed = 2;
     } else /* state->charsOnStack == 2 */ {
       firstSet[0] = state->c[0];
       firstSet[1] = state->c[1];
       firstSet[2] = src[0];
       firstSet[3] = '\0';
+      consumed = 1;
     }
+
     Encode(firstSet, 3, state->buffer);
     state->buffer += 4;
-    countRemaining -= (3 - state->charsOnStack);
-    src += (3 - state->charsOnStack);
+    countRemaining -= consumed;
+    src += consumed;
     state->charsOnStack = 0;
+
+    // Bail if there is nothing left.
+    if (!countRemaining) {
+      return NS_OK;
+    }
   }
 
-  // Encode the bulk of the
+  // Encode as many full triplets as possible.
   uint32_t encodeLength = countRemaining - countRemaining % 3;
   MOZ_ASSERT(encodeLength % 3 == 0,
              "Should have an exact number of triplets!");
@@ -140,9 +161,6 @@ EncodeInputStream_Encoder(nsIInputStream* aStream,
   src += encodeLength;
   countRemaining -= encodeLength;
 
-  // We must consume all data, so if there's some data left stash it
-  *aWriteCount = aCount;
-
   if (countRemaining) {
     // We should never have a full triplet left at this point.
     MOZ_ASSERT(countRemaining < 3, "We should have encoded more!");