diff --git a/src/vs/editor/common/model.ts b/src/vs/editor/common/model.ts
index 31f7b8be66885..bf6ecbbee7c1b 100644
--- a/src/vs/editor/common/model.ts
+++ b/src/vs/editor/common/model.ts
@@ -666,7 +666,7 @@ export interface ITextModel {
 	 * @param limitResultCount Limit the number of results
 	 * @return The ranges where the matches are. It is empty if not matches have been found.
 	 */
-	findMatches(searchString: string, searchOnlyEditableRange: boolean, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean, limitResultCount?: number): FindMatch[];
+	findMatches(searchString: string, searchOnlyEditableRange: boolean, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean, limitResultCount?: number): FindMatch[];
 	/**
 	 * Search the model.
 	 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -678,7 +678,7 @@ export interface ITextModel {
 	 * @param limitResultCount Limit the number of results
 	 * @return The ranges where the matches are. It is empty if no matches have been found.
 	 */
-	findMatches(searchString: string, searchScope: IRange, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean, limitResultCount?: number): FindMatch[];
+	findMatches(searchString: string, searchScope: IRange, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean, limitResultCount?: number): FindMatch[];
 	/**
 	 * Search the model for the next match. Loops to the beginning of the model if needed.
 	 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -689,7 +689,7 @@ export interface ITextModel {
 	 * @param captureMatches The result will contain the captured groups.
 	 * @return The range where the next match is. It is null if no next match has been found.
 	 */
-	findNextMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean): FindMatch;
+	findNextMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean): FindMatch;
 	/**
 	 * Search the model for the previous match. Loops to the end of the model if needed.
 	 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -700,7 +700,7 @@ export interface ITextModel {
 	 * @param captureMatches The result will contain the captured groups.
 	 * @return The range where the previous match is. It is null if no previous match has been found.
 	 */
-	findPreviousMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean): FindMatch;
+	findPreviousMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean): FindMatch;
 
 	/**
 	 * Get the language associated with this model.
diff --git a/src/vs/monaco.d.ts b/src/vs/monaco.d.ts
index 9165cba0dc64d..6396279982ec5 100644
--- a/src/vs/monaco.d.ts
+++ b/src/vs/monaco.d.ts
@@ -1546,7 +1546,7 @@ declare module monaco.editor {
 		 * @param limitResultCount Limit the number of results
 		 * @return The ranges where the matches are. It is empty if not matches have been found.
 		 */
-		findMatches(searchString: string, searchOnlyEditableRange: boolean, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean, limitResultCount?: number): FindMatch[];
+		findMatches(searchString: string, searchOnlyEditableRange: boolean, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean, limitResultCount?: number): FindMatch[];
 		/**
 		 * Search the model.
 		 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -1558,7 +1558,7 @@ declare module monaco.editor {
 		 * @param limitResultCount Limit the number of results
 		 * @return The ranges where the matches are. It is empty if no matches have been found.
 		 */
-		findMatches(searchString: string, searchScope: IRange, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean, limitResultCount?: number): FindMatch[];
+		findMatches(searchString: string, searchScope: IRange, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean, limitResultCount?: number): FindMatch[];
 		/**
 		 * Search the model for the next match. Loops to the beginning of the model if needed.
 		 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -1569,7 +1569,7 @@ declare module monaco.editor {
 		 * @param captureMatches The result will contain the captured groups.
 		 * @return The range where the next match is. It is null if no next match has been found.
 		 */
-		findNextMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean): FindMatch;
+		findNextMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean): FindMatch;
 		/**
 		 * Search the model for the previous match. Loops to the end of the model if needed.
 		 * @param searchString The string used to search. If it is a regular expression, set `isRegex` to true.
@@ -1580,7 +1580,7 @@ declare module monaco.editor {
 		 * @param captureMatches The result will contain the captured groups.
 		 * @return The range where the previous match is. It is null if no previous match has been found.
 		 */
-		findPreviousMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string, captureMatches: boolean): FindMatch;
+		findPreviousMatch(searchString: string, searchStart: IPosition, isRegex: boolean, matchCase: boolean, wordSeparators: string | null, captureMatches: boolean): FindMatch;
 		/**
 		 * Get the language associated with this model.
 		 */