diff --git a/packages/sdk38/src/coins.spec.ts b/packages/sdk38/src/coins.spec.ts
new file mode 100644
index 00000000..91483913
--- /dev/null
+++ b/packages/sdk38/src/coins.spec.ts
@@ -0,0 +1,35 @@
+import { coin, coins } from "./coins";
+
+describe("coins", () => {
+  describe("coin", () => {
+    it("works for basic values", () => {
+      expect(coin(123, "utoken")).toEqual({ amount: "123", denom: "utoken" });
+      expect(coin(123.0, "utoken")).toEqual({ amount: "123", denom: "utoken" });
+      expect(coin(Number.MAX_SAFE_INTEGER, "utoken")).toEqual({
+        amount: "9007199254740991",
+        denom: "utoken",
+      });
+      expect(coin(+0, "utoken")).toEqual({ amount: "0", denom: "utoken" });
+      expect(coin(-0, "utoken")).toEqual({ amount: "0", denom: "utoken" });
+    });
+
+    it("throws for non-safe-integer values", () => {
+      expect(() => coin(1.23, "utoken")).toThrow();
+      expect(() => coin(NaN, "utoken")).toThrow();
+      expect(() => coin(Number.POSITIVE_INFINITY, "utoken")).toThrow();
+      expect(() => coin(Number.MAX_SAFE_INTEGER + 1, "utoken")).toThrow();
+    });
+
+    it("throws for negative values", () => {
+      expect(() => coin(-1, "utoken")).toThrow();
+      expect(() => coin(Number.MIN_SAFE_INTEGER, "utoken")).toThrow();
+      expect(() => coin(Number.NEGATIVE_INFINITY, "utoken")).toThrow();
+    });
+  });
+
+  describe("coins", () => {
+    it("returns one element array of coin", () => {
+      expect(coins(123, "utoken")).toEqual([{ amount: "123", denom: "utoken" }]);
+    });
+  });
+});
diff --git a/packages/sdk38/src/coins.ts b/packages/sdk38/src/coins.ts
index 62f5f9ce..f14f996a 100644
--- a/packages/sdk38/src/coins.ts
+++ b/packages/sdk38/src/coins.ts
@@ -1,3 +1,5 @@
+import { Uint53 } from "@cosmjs/math";
+
 export interface Coin {
   readonly denom: string;
   readonly amount: string;
@@ -5,7 +7,7 @@ export interface Coin {
 
 /** Creates a coin */
 export function coin(amount: number, denom: string): Coin {
-  return { amount: amount.toString(), denom: denom };
+  return { amount: new Uint53(amount).toString(), denom: denom };
 }
 
 /** Creates a list of coins with one element */