diff --git a/types/gm/gm-tests.ts b/types/gm/gm-tests.ts
index 9f59854467..b2efd9505a 100644
--- a/types/gm/gm-tests.ts
+++ b/types/gm/gm-tests.ts
@@ -62,6 +62,7 @@ declare const font: string;
 declare const quality: number;
 declare const align: string;
 declare const depth: number;
+declare const defineValue: string;
 let readStream: stream.PassThrough;
 
 gm(src)
@@ -104,7 +105,7 @@ gm(src)
 	.crop(width, height, x, y, usePercent)
 	.cycle(factor)
 	.deconstruct()
-	.define()
+	.define(defineValue)
 	.delay(time)
 	.density(width, height)
 	.despeckle()
diff --git a/types/gm/index.d.ts b/types/gm/index.d.ts
index 0670f0ae0f..5e8853efce 100644
--- a/types/gm/index.d.ts
+++ b/types/gm/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for gm 1.17
+// Type definitions for gm 1.18
 // Project: https://github.com/aheckmann/gm
 // Definitions by: Joel Spadin <https://github.com/ChaosinaCan>, Maarten van Vliet <https://github.com/maartenvanvliet>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
@@ -123,7 +123,7 @@ declare namespace m {
         crop(width: number, height: number, x?: number, y?: number, percent?: boolean): State;
         cycle(amount: number): State;
         deconstruct(): State;
-        define(): State;
+        define(value: string): State;
         delay(milliseconds: number): State;
         density(width: number, height: number): State;
         despeckle(): State;