diff --git a/types/ps-tree/index.d.ts b/types/ps-tree/index.d.ts
index 7dad700315..a915c9f547 100644
--- a/types/ps-tree/index.d.ts
+++ b/types/ps-tree/index.d.ts
@@ -10,7 +10,7 @@ declare namespace ps_tree {
         PID: string;
         STAT: string;
     }
-    
+
     const prototype: {
     };
 }
diff --git a/types/ps-tree/ps-tree-tests.ts b/types/ps-tree/ps-tree-tests.ts
index 308900e6e2..9af8b9f53c 100644
--- a/types/ps-tree/ps-tree-tests.ts
+++ b/types/ps-tree/ps-tree-tests.ts
@@ -1,8 +1,9 @@
 import * as ps_tree from "ps-tree";
 
-ps_tree(1, (error: Error, children: ps_tree.PS[]) => {
+ps_tree(1, () => {}); // $ExpectType void
+ps_tree(1, (error: Error, children: ReadonlyArray<ps_tree.PS>) => {
     children[0].COMMAND; // $ExpectType string
     children[0].PID; // $ExpectType string
     children[0].PPID; // $ExpectType string
     children[0].STAT; // $ExpectType string
-}); // $ExpectType void
+});
diff --git a/types/ps-tree/tsconfig.json b/types/ps-tree/tsconfig.json
index b8a7b33b11..4f3bd8dcfc 100644
--- a/types/ps-tree/tsconfig.json
+++ b/types/ps-tree/tsconfig.json
@@ -6,6 +6,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
+        "strictFunctionTypes": true,
         "strictNullChecks": true,
         "baseUrl": "../",
         "typeRoots": [