diff --git a/RELEASE-NOTES-1.21 b/RELEASE-NOTES-1.21
index 85c01f4e188..4afda19414c 100644
--- a/RELEASE-NOTES-1.21
+++ b/RELEASE-NOTES-1.21
@@ -123,6 +123,7 @@ production.
   log when determining whether an account is used.
 * (bug 43379) Gracefully fail if rev_len is unavailable for a revision on the
   History page.
+* (bug 42949) API no longer assumes all exceptions are MWException.
 
 === API changes in 1.21 ===
 * prop=revisions can now report the contentmodel and contentformat.
diff --git a/includes/api/ApiMain.php b/includes/api/ApiMain.php
index 97f5ecd61ea..1e8c84edce3 100644
--- a/includes/api/ApiMain.php
+++ b/includes/api/ApiMain.php
@@ -375,7 +375,7 @@ class ApiMain extends ApiBase {
 			wfRunHooks( 'ApiMain::onException', array( $this, $e ) );
 
 			// Log it
-			if ( !( $e instanceof UsageException ) ) {
+			if ( $e instanceof MWException && !( $e instanceof UsageException ) ) {
 				global $wgLogExceptionBacktrace;
 				if ( $wgLogExceptionBacktrace ) {
 					wfDebugLog( 'exception', $e->getLogMessage() . "\n" . $e->getTraceAsString() . "\n" );