diff --git a/gfx/layers/LayerScope.cpp b/gfx/layers/LayerScope.cpp
index 95669cf363b7..06d05c73358b 100644
--- a/gfx/layers/LayerScope.cpp
+++ b/gfx/layers/LayerScope.cpp
@@ -1477,8 +1477,10 @@ LayerScopeWebSocketManager::SocketHandler::WebSocketHandshake(nsTArray<nsCString
     uint8_t digest[SHA1Sum::kHashSize]; // SHA1 digests are 20 bytes long.
     sha1.finish(digest);
     nsCString newString(reinterpret_cast<char*>(digest), SHA1Sum::kHashSize);
-    Base64Encode(newString, res);
-
+    rv = Base64Encode(newString, res);
+    if (NS_FAILED(rv)) {
+        return false;
+    }
     nsCString response("HTTP/1.1 101 Switching Protocols\r\n");
     response.AppendLiteral("Upgrade: websocket\r\n");
     response.AppendLiteral("Connection: Upgrade\r\n");
diff --git a/netwerk/system/linux/nsNotifyAddrListener_Linux.cpp b/netwerk/system/linux/nsNotifyAddrListener_Linux.cpp
index 0c5315921ffc..c4141dc5be3b 100644
--- a/netwerk/system/linux/nsNotifyAddrListener_Linux.cpp
+++ b/netwerk/system/linux/nsNotifyAddrListener_Linux.cpp
@@ -180,7 +180,8 @@ void nsNotifyAddrListener::calculateNetworkId(void)
                                 sha1.finish(digest);
                                 nsCString newString(reinterpret_cast<char*>(digest),
                                                     SHA1Sum::kHashSize);
-                                Base64Encode(newString, output);
+                                nsresult rv = Base64Encode(newString, output);
+                                MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
                                 LOG(("networkid: id %s\n", output.get()));
                                 if (mNetworkId != output) {
                                     // new id
diff --git a/netwerk/system/mac/nsNetworkLinkService.mm b/netwerk/system/mac/nsNetworkLinkService.mm
index f499f1e3aba9..2901cab935a5 100644
--- a/netwerk/system/mac/nsNetworkLinkService.mm
+++ b/netwerk/system/mac/nsNetworkLinkService.mm
@@ -270,7 +270,8 @@ void nsNetworkLinkService::calculateNetworkId(void)
         sha1.finish(digest);
         nsCString newString(reinterpret_cast<char*>(digest),
                             SHA1Sum::kHashSize);
-        Base64Encode(newString, output);
+        nsresult rv = Base64Encode(newString, output);
+        MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
         LOG(("networkid: id %s\n", output.get()));
         mNetworkId = output;
     }
diff --git a/xpcom/io/Base64.h b/xpcom/io/Base64.h
index e5c930d8fa19..e48b1fbbfd52 100644
--- a/xpcom/io/Base64.h
+++ b/xpcom/io/Base64.h
@@ -13,27 +13,27 @@ class nsIInputStream;
 
 namespace mozilla {
 
-nsresult
+MOZ_MUST_USE nsresult
 Base64EncodeInputStream(nsIInputStream* aInputStream,
                         nsACString& aDest,
                         uint32_t aCount,
                         uint32_t aOffset = 0);
-nsresult
+MOZ_MUST_USE nsresult
 Base64EncodeInputStream(nsIInputStream* aInputStream,
                         nsAString& aDest,
                         uint32_t aCount,
                         uint32_t aOffset = 0);
 
-nsresult
+MOZ_MUST_USE nsresult
 Base64Encode(const char* aBinary, uint32_t aBinaryLen, char** aBase64);
-nsresult
+MOZ_MUST_USE nsresult
 Base64Encode(const nsACString& aBinary, nsACString& aBase64);
-nsresult
+MOZ_MUST_USE nsresult
 Base64Encode(const nsAString& aBinary, nsAString& aBase64);
 
-nsresult
+MOZ_MUST_USE nsresult
 Base64Decode(const nsACString& aBase64, nsACString& aBinary);
-nsresult
+MOZ_MUST_USE nsresult
 Base64Decode(const nsAString& aBase64, nsAString& aBinary);
 
 enum class Base64URLEncodePaddingPolicy {
@@ -46,7 +46,7 @@ enum class Base64URLEncodePaddingPolicy {
  * Aims to encode the data in constant time. The caller retains ownership
  * of |aBinary|.
  */
-nsresult
+MOZ_MUST_USE nsresult
 Base64URLEncode(uint32_t aBinaryLen, const uint8_t* aBinary,
                 Base64URLEncodePaddingPolicy aPaddingPolicy,
                 nsACString& aBase64);
@@ -60,7 +60,7 @@ enum class Base64URLDecodePaddingPolicy {
 /**
  * Decodes a Base64 URL-encoded |aBase64| into |aBinary|.
  */
-nsresult
+MOZ_MUST_USE nsresult
 Base64URLDecode(const nsACString& aBase64,
                 Base64URLDecodePaddingPolicy aPaddingPolicy,
                 FallibleTArray<uint8_t>& aBinary);