Index: wp-includes/js/tinymce/plugins/wpeditimage/js/editimage.dev.js
===================================================================
--- wp-includes/js/tinymce/plugins/wpeditimage/js/editimage.dev.js	(revision 19654)
+++ wp-includes/js/tinymce/plugins/wpeditimage/js/editimage.dev.js	(working copy)
@@ -362,7 +362,17 @@
 			return;
 		}
 
-		if ( f.img_cap.value != '' && f.width.value != '' ) {
+		// Reset width if it's not a valid number
+		if ( !f.width.value.match(/^\d+$/) ) {
+			f.width.value = el.width;
+		}
+
+		// Reset width if it's not a valid number
+		if ( !f.height.value.match(/^\d+$/) ) {
+			f.height.value = el.height
+		}
+
+		if ( f.img_cap.value != '' ) {
 			do_caption = 1;
 			img_class = img_class.replace( /align[^ "']+\s?/gi, '' );
 		}