diff --git a/d3/d3-tests.ts b/d3/d3-tests.ts
index 923e439e2e..fca389b02a 100644
--- a/d3/d3-tests.ts
+++ b/d3/d3-tests.ts
@@ -788,7 +788,7 @@ function populationPyramid() {
         // Produce a map from year and birthyear to [male, female].
         data = d3.nest()
             .key(function (d) { return d.year; } )
-            .key(function (d) { return d.year - d.age; } )
+            .key(function (d) { return '' + (d.year - d.age); } )
             .rollup(function (v) { return v.map(function (d) { return d.people; } ); } )
             .map(data);
 
diff --git a/d3/d3.d.ts b/d3/d3.d.ts
index a4fbd690a0..b12dc01c1c 100644
--- a/d3/d3.d.ts
+++ b/d3/d3.d.ts
@@ -755,7 +755,7 @@ declare module D3 {
     }
 
     export interface Nest {
-        key(keyFunction: (data: any, index: number) => any): Nest;
+        key(keyFunction: (data: any, index: number) => string): Nest;
         sortKeys(comparator: (d1: any, d2: any) => number): Nest;
         sortValues(comparator: (d1: any, d2: any) => number): Nest;
         rollup(rollupFunction: (data: any, index: number) => any): Nest;