--- a/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
+++ b/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
@@ -5,16 +5,18 @@
 #include <algorithm>
 #include <assert.h>
 #include <ctype.h>
 #include <stdarg.h>
 #include <stdint.h>
 #include <vector>
 
 #include "ClearKeyUtils.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
 #include "mozilla/Endian.h"
 #include "mozilla/NullPtr.h"
 #include "openaes/oaes_lib.h"
 
 using namespace std;
 
 #define FOURCC(a,b,c,d) ((a << 24) + (b << 16) + (c << 8) + d)
 
@@ -101,17 +103,22 @@ EncodeBase64Web(vector<uint8_t> aBinary,
       data++;
     } else {
       out[i] = 0;
     }
 
     out[i] += (*data >> (shift + 2)) & sMask;
     shift = (shift + 2) % 8;
 
-    out[i] = sAlphabet[out[i]];
+    auto idx = out[i];
+    MOZ_ASSERT(idx >= 0 &&
+               static_cast<size_t>(idx) < MOZ_ARRAY_LENGTH(sAlphabet),
+               "out of bounds index for 'sAlphabet'");
+    // Cast idx to size_t to pacify clang 'Wchar-subscripts' warning:
+    out[i] = sAlphabet[static_cast<size_t>(idx)];
   }
 
   return true;
 }
 
 /* static */ void
 ClearKeyUtils::ParseInitData(const uint8_t* aInitData, uint32_t aInitDataSize,
                              vector<KeyId>& aOutKeys)