diff --git a/modules/libjar/nsJARChannel.cpp b/modules/libjar/nsJARChannel.cpp
index 059c2287112a..9e1a3cff7ca3 100644
--- a/modules/libjar/nsJARChannel.cpp
+++ b/modules/libjar/nsJARChannel.cpp
@@ -77,6 +77,7 @@ class nsJARInputThunk : public nsIInputStream {
         mJarReader(zipReader),
         mJarEntry(jarEntry),
         mContentLength(-1) {
+    MOZ_DIAGNOSTIC_ASSERT(zipReader, "zipReader must not be null");
     if (ENTRY_IS_DIRECTORY(mJarEntry) && fullJarURI) {
       nsCOMPtr<nsIURI> urlWithoutQueryRef;
       nsresult rv = NS_MutateURI(fullJarURI)
@@ -110,6 +111,9 @@ class nsJARInputThunk : public nsIInputStream {
 NS_IMPL_ISUPPORTS(nsJARInputThunk, nsIInputStream)
 
 nsresult nsJARInputThunk::Init() {
+  if (!mJarReader) {
+    return NS_ERROR_INVALID_ARG;
+  }
   nsresult rv;
   if (ENTRY_IS_DIRECTORY(mJarEntry)) {
     // A directory stream also needs the Spec of the FullJarURI
diff --git a/modules/libjar/nsJARInputStream.cpp b/modules/libjar/nsJARInputStream.cpp
index ae202724d0d9..a47bd7a09fd7 100644
--- a/modules/libjar/nsJARInputStream.cpp
+++ b/modules/libjar/nsJARInputStream.cpp
@@ -35,7 +35,10 @@ NS_IMPL_ISUPPORTS(nsJARInputStream, nsIInputStream)
 nsresult nsJARInputStream::InitFile(nsZipHandle* aFd, const uint8_t* aData,
                                     nsZipItem* aItem) {
   nsresult rv = NS_OK;
-  MOZ_ASSERT(aFd, "Argument may not be null");
+  MOZ_DIAGNOSTIC_ASSERT(aFd, "Argument may not be null");
+  if (!aFd) {
+    return NS_ERROR_INVALID_ARG;
+  }
   MOZ_ASSERT(aItem, "Argument may not be null");
 
   // Mark it as closed, in case something fails in initialisation
diff --git a/modules/libjar/nsZipArchive.cpp b/modules/libjar/nsZipArchive.cpp
index 1dcfd57c6595..1f43a0d6259c 100644
--- a/modules/libjar/nsZipArchive.cpp
+++ b/modules/libjar/nsZipArchive.cpp
@@ -766,6 +766,7 @@ nsZipHandle* nsZipArchive::GetFD() const { return mFd.get(); }
 //---------------------------------------------
 uint32_t nsZipArchive::GetDataOffset(nsZipItem* aItem) {
   MOZ_ASSERT(aItem);
+  MOZ_DIAGNOSTIC_ASSERT(mFd);
 
   uint32_t offset;
   MMAP_FAULT_HANDLER_BEGIN_HANDLE(mFd)
@@ -809,7 +810,10 @@ uint32_t nsZipArchive::GetDataOffset(nsZipItem* aItem) {
 // nsZipArchive::GetData
 //---------------------------------------------
 const uint8_t* nsZipArchive::GetData(nsZipItem* aItem) {
-  MOZ_ASSERT(aItem);
+  MOZ_DIAGNOSTIC_ASSERT(aItem);
+  if (!aItem) {
+    return nullptr;
+  }
   uint32_t offset = GetDataOffset(aItem);
 
   MMAP_FAULT_HANDLER_BEGIN_HANDLE(mFd)
@@ -838,6 +842,7 @@ nsZipArchive::nsZipArchive(nsZipHandle* aZipHandle, PRFileDesc* aFd,
     : mRefCnt(0), mFd(aZipHandle), mUseZipLog(false), mBuiltSynthetics(false) {
   // initialize the table to nullptr
   memset(mFiles, 0, sizeof(mFiles));
+  MOZ_DIAGNOSTIC_ASSERT(aZipHandle);
 
   //-- get table of contents for archive
   aRv = BuildFileList(aFd);