diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d7f2085..9354f96 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -31,16 +31,16 @@ jobs:
         node-version: [14.x, 16.x, 18.x]
         opa-version:
         - 0.30.2 # last version with ABI 1.1, 0.31.0+ has ABI 1.2
-        - 0.41.0 # 0.35.0 is the first release with https://github.com/open-policy-agent/opa/pull/4055
+        - 0.45.0 # 0.35.0 is the first release with https://github.com/open-policy-agent/opa/pull/4055
 
     steps:
     - uses: actions/checkout@v3
 
-    - name: Checkout OPA v${{ matrix.opa-version }}
+    - name: Checkout OPA v0.45.0 testcases
       uses: actions/checkout@v3
       with:
         repository: open-policy-agent/opa
-        ref: v${{ matrix.opa-version }}
+        ref: v0.45.0
         path: opa
 
     - run: mkdir test/cases
diff --git a/package.json b/package.json
index 586e4df..5692fe5 100644
--- a/package.json
+++ b/package.json
@@ -18,7 +18,7 @@
     "lint": "git ls-files | xargs deno lint",
     "fmt:check": "git ls-files | xargs deno fmt --check",
     "fmt": "git ls-files | xargs deno fmt",
-    "test": "jest --verbose"
+    "test": "jest"
   },
   "repository": {
     "type": "git",
diff --git a/src/builtins/json.js b/src/builtins/json.js
index 74acc75..df63490 100644
--- a/src/builtins/json.js
+++ b/src/builtins/json.js
@@ -1,6 +1,6 @@
 function isValidJSON(str) {
   if (typeof str !== "string") {
-    return;
+    return false;
   }
   try {
     JSON.parse(str);
diff --git a/src/builtins/yaml.js b/src/builtins/yaml.js
index e12f1e3..beb5d1b 100644
--- a/src/builtins/yaml.js
+++ b/src/builtins/yaml.js
@@ -30,9 +30,8 @@ function parse(str) {
 }
 
 module.exports = {
-  // is_valid is expected to return nothing if input is invalid otherwise
-  // true/false for it being valid YAML.
-  "yaml.is_valid": (str) => typeof str === "string" ? parse(str).ok : undefined,
+  // is_valid is expected to return false if input is invalid; and true/false for it being valid YAML.
+  "yaml.is_valid": (str) => typeof str === "string" && parse(str).ok,
   "yaml.marshal": (data) => yaml.stringify(data),
   "yaml.unmarshal": (str) => parse(str).result,
 };