diff --git a/src/core/operations/BLAKE3.mjs b/src/core/operations/BLAKE3.mjs
index a22eb0b8..8002a277 100644
--- a/src/core/operations/BLAKE3.mjs
+++ b/src/core/operations/BLAKE3.mjs
@@ -51,6 +51,11 @@ class BLAKE3 extends Operation {
     run(input, args) {
         const key = args[1];
         const size = args[0];
+
+        if (!Number.isInteger(size) || size < 0) {
+            throw new OperationError("Size must be a non-negative integer");
+        }
+
         const opts = { dkLen: size };
         const inputBytes = new Uint8Array(Utils.strToArrayBuffer(input));
         if (key !== "") {
diff --git a/tests/operations/tests/BLAKE3.mjs b/tests/operations/tests/BLAKE3.mjs
index e15144b2..92504ca9 100644
--- a/tests/operations/tests/BLAKE3.mjs
+++ b/tests/operations/tests/BLAKE3.mjs
@@ -34,6 +34,24 @@ TestRegister.addTests([
                 "args": [32, ""] }
         ]
     },
+    {
+        name: "BLAKE3: negative size",
+        input: "hello",
+        expectedOutput: "Size must be a non-negative integer",
+        recipeConfig: [
+            { "op": "BLAKE3",
+                "args": [-6, ""] }
+        ]
+    },
+    {
+        name: "BLAKE3: fractional size",
+        input: "hello",
+        expectedOutput: "Size must be a non-negative integer",
+        recipeConfig: [
+            { "op": "BLAKE3",
+                "args": [1.2, ""] }
+        ]
+    },
     {
         name: "BLAKE3: Key Test",
         input: "Hello world",