diff --git a/mozilla/string/src/nsSharableString.cpp b/mozilla/string/src/nsSharableString.cpp
index 809b7c18f91..53f4b5a20f0 100644
--- a/mozilla/string/src/nsSharableString.cpp
+++ b/mozilla/string/src/nsSharableString.cpp
@@ -124,8 +124,9 @@ nsSharableString::do_AssignFromReadable( const abstract_string_type& aReadable )
       {
         // null-check |mBuffer.get()| here only for the constructor
         // taking |const abstract_string_type&|
+        size_type inLength;
         if ( mBuffer.get() && !mBuffer->IsShared() &&
-             mBuffer->StorageLength() > aReadable.Length() &&
+             mBuffer->StorageLength() > (inLength = aReadable.Length()) &&
              !aReadable.IsDependentOn(*this) )
           {
             abstract_string_type::const_iterator fromBegin, fromEnd;
@@ -133,6 +134,7 @@ nsSharableString::do_AssignFromReadable( const abstract_string_type& aReadable )
             *copy_string( aReadable.BeginReading(fromBegin),
                           aReadable.EndReading(fromEnd),
                           storage_start ) = char_type(0);
+            mBuffer->DataEnd(storage_start + inLength);
             return; // don't want to assign to |mBuffer| below
           }
         else
@@ -268,8 +270,9 @@ nsSharableCString::do_AssignFromReadable( const abstract_string_type& aReadable
       {
         // null-check |mBuffer.get()| here only for the constructor
         // taking |const abstract_string_type&|
+        size_type inLength;
         if ( mBuffer.get() && !mBuffer->IsShared() &&
-             mBuffer->StorageLength() > aReadable.Length() &&
+             mBuffer->StorageLength() > (inLength = aReadable.Length()) &&
              !aReadable.IsDependentOn(*this) )
           {
             abstract_string_type::const_iterator fromBegin, fromEnd;
@@ -277,6 +280,7 @@ nsSharableCString::do_AssignFromReadable( const abstract_string_type& aReadable
             *copy_string( aReadable.BeginReading(fromBegin),
                           aReadable.EndReading(fromEnd),
                           storage_start ) = char_type(0);
+            mBuffer->DataEnd(storage_start + inLength);
             return; // don't want to assign to |mBuffer| below
           }
         else
diff --git a/mozilla/xpcom/string/src/nsSharableString.cpp b/mozilla/xpcom/string/src/nsSharableString.cpp
index 809b7c18f91..53f4b5a20f0 100644
--- a/mozilla/xpcom/string/src/nsSharableString.cpp
+++ b/mozilla/xpcom/string/src/nsSharableString.cpp
@@ -124,8 +124,9 @@ nsSharableString::do_AssignFromReadable( const abstract_string_type& aReadable )
       {
         // null-check |mBuffer.get()| here only for the constructor
         // taking |const abstract_string_type&|
+        size_type inLength;
         if ( mBuffer.get() && !mBuffer->IsShared() &&
-             mBuffer->StorageLength() > aReadable.Length() &&
+             mBuffer->StorageLength() > (inLength = aReadable.Length()) &&
              !aReadable.IsDependentOn(*this) )
           {
             abstract_string_type::const_iterator fromBegin, fromEnd;
@@ -133,6 +134,7 @@ nsSharableString::do_AssignFromReadable( const abstract_string_type& aReadable )
             *copy_string( aReadable.BeginReading(fromBegin),
                           aReadable.EndReading(fromEnd),
                           storage_start ) = char_type(0);
+            mBuffer->DataEnd(storage_start + inLength);
             return; // don't want to assign to |mBuffer| below
           }
         else
@@ -268,8 +270,9 @@ nsSharableCString::do_AssignFromReadable( const abstract_string_type& aReadable
       {
         // null-check |mBuffer.get()| here only for the constructor
         // taking |const abstract_string_type&|
+        size_type inLength;
         if ( mBuffer.get() && !mBuffer->IsShared() &&
-             mBuffer->StorageLength() > aReadable.Length() &&
+             mBuffer->StorageLength() > (inLength = aReadable.Length()) &&
              !aReadable.IsDependentOn(*this) )
           {
             abstract_string_type::const_iterator fromBegin, fromEnd;
@@ -277,6 +280,7 @@ nsSharableCString::do_AssignFromReadable( const abstract_string_type& aReadable
             *copy_string( aReadable.BeginReading(fromBegin),
                           aReadable.EndReading(fromEnd),
                           storage_start ) = char_type(0);
+            mBuffer->DataEnd(storage_start + inLength);
             return; // don't want to assign to |mBuffer| below
           }
         else