diff --git a/js/src/asmjs/Wasm.cpp b/js/src/asmjs/Wasm.cpp
index 0dd640414ac5..b43c315188b9 100644
--- a/js/src/asmjs/Wasm.cpp
+++ b/js/src/asmjs/Wasm.cpp
@@ -312,7 +312,7 @@ DecodeDeclarationSection(JSContext* cx, Decoder& d, ModuleGeneratorData* init)
         if (!d.readVarU32(&sigIndex))
             return Fail(cx, d, "expected declaration signature index");
 
-        if (sigIndex > init->sigs.length())
+        if (sigIndex >= init->sigs.length())
             return Fail(cx, d, "declaration signature index out of range");
 
         init->funcSigs[i] = &init->sigs[sigIndex];
diff --git a/js/src/jit-test/tests/wasm/binary.js b/js/src/jit-test/tests/wasm/binary.js
index b55912e1b01c..999ee0a237fb 100644
--- a/js/src/jit-test/tests/wasm/binary.js
+++ b/js/src/jit-test/tests/wasm/binary.js
@@ -108,3 +108,15 @@ wasmEval(toBuf(moduleWithSections([sigSection([{args:[I32Code], ret:VoidCode}])]
 
 assertErrorMessage(() => wasmEval(toBuf(moduleWithSections([sigSection([{args:[], ret:100}])]))), Error, /bad expression type/);
 assertErrorMessage(() => wasmEval(toBuf(moduleWithSections([sigSection([{args:[100], ret:VoidCode}])]))), Error, /bad value type/);
+
+function declSection(decls) {
+    var body = [];
+    body.push(...varU32(decls.length));
+    for (var decl of decls)
+        body.push(...varU32(decl));
+    return { name: declSectionStr, body };
+}
+
+assertThrowsInstanceOf(() => wasmEval(toBuf(moduleWithSections([sigSection([]), declSection([0])]))), Error, /signature index out of range/);
+assertThrowsInstanceOf(() => wasmEval(toBuf(moduleWithSections([sigSection([{args:[], ret:VoidCode}]), declSection([1])]))), Error, /signature index out of range/);
+wasmEval(toBuf(moduleWithSections([sigSection([{args:[], ret:VoidCode}]), declSection([0])])));