diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index d460e82..2e9d5bf 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -17,37 +17,43 @@ jobs:
             name: "Ubuntu Latest Release",
             os: "ubuntu-latest",
             build-type: Release,
-            dependencies: "sudo apt install ninja-build"
+            dependencies: "sudo apt install ninja-build",
+            defines: ""
           }
         - {
             name: "Ubuntu 20.04 Release",
             os: "ubuntu-20.04",
             build-type: Release,
-            dependencies: "sudo apt install ninja-build"
+            dependencies: "sudo apt install ninja-build",
+            defines: "-DREGOCPP_USE_CXX17=ON"
           }
         - {
             name: "macOS Release",
             os: "macos-latest",
             build-type: Release,
-            dependencies: "brew update && brew install cmake ninja"
+            dependencies: "brew update && brew install cmake ninja",
+            defines: ""
           }
         - {
             name: "Ubuntu Latest Debug",
             os: "ubuntu-latest",
             build-type: Debug,
-            dependencies: "sudo apt install ninja-build"
+            dependencies: "sudo apt install ninja-build",
+            defines: ""
           }
         - {
             name: "Ubuntu 20.04 Debug",
             os: "ubuntu-20.04",
             build-type: Debug,
-            dependencies: "sudo apt install ninja-build"
+            dependencies: "sudo apt install ninja-build",
+            defines: "-DREGOCPP_USE_CXX17=ON"
           }
         - {
             name: "macOS Debug",
             os: "macos-latest",
             build-type: Debug,
-            dependencies: "brew update && brew install cmake ninja"
+            dependencies: "brew update && brew install cmake ninja",
+            defines: ""
           }
     runs-on: ${{ matrix.config.os }}
     name: ${{ matrix.config.name }}
@@ -56,7 +62,7 @@ jobs:
     - name: Install ninja
       run: ${{ matrix.config.dependencies }}
     - name: Configure CMake
-      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.config.build-type}} -G Ninja
+      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.config.build-type}} ${{matrix.config.defines}} -G Ninja
     - name: Build
       working-directory: ${{github.workspace}}/build
       # Build your program with the given configuration
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4d1b3c0..9bde60a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,7 +12,7 @@ endif ()
 if (DEFINED ENV{REGOCPP_TAG})
   set(REGOCPP_TAG $ENV{REGOCPP_TAG})
 else ()
-  set(REGOCPP_TAG "cb967637dbf7cee25117203bbdf9c10b62dfb25a")
+  set(REGOCPP_TAG "cfabe01a63e1d302e3924d03531aa0ad41e8947b")
 endif()
 
 include(FetchContent)
diff --git a/Tests/compartment_callers.query.expected b/Tests/compartment_callers.query.expected
index f84f071..8736f87 100644
--- a/Tests/compartment_callers.query.expected
+++ b/Tests/compartment_callers.query.expected
@@ -1 +1 @@
-["allocator_test", "test_runner"]
+["allocator_test","test_runner"]
diff --git a/Tests/undefined.query b/Tests/undefined.query
new file mode 100644
index 0000000..8f71e3f
--- /dev/null
+++ b/Tests/undefined.query
@@ -0,0 +1,2 @@
+# Check that invalid queries return undefined
+--board inputs/sail.json -j inputs/test-suite.json -q 'data.this.is.undefined'
diff --git a/Tests/undefined.query.expected b/Tests/undefined.query.expected
new file mode 100644
index 0000000..417b7b5
--- /dev/null
+++ b/Tests/undefined.query.expected
@@ -0,0 +1 @@
+undefined
diff --git a/audit.cc b/audit.cc
index f2e46ba..0bbd747 100644
--- a/audit.cc
+++ b/audit.cc
@@ -243,6 +243,73 @@ namespace
 		return scalar(std::move(result));
 	}
 
+	std::string
+	extract_first_expression_from_result(const std::string &result_json)
+	{
+		if (result_json == "undefined")
+		{
+			return result_json;
+		}
+
+		nlohmann::json result;
+		try
+		{
+			result = result.parse(result_json);
+		}
+		catch (nlohmann::json::parse_error &e)
+		{
+			return e.what();
+		}
+
+		if (result.is_array())
+		{
+			if (result.empty())
+			{
+				std::cerr << "warning: query returned no results." << std::endl;
+				return result_json;
+			}
+
+			if (result.size() > 1)
+			{
+				std::cerr << "warning: query returned multiple results. Only "
+				             "the first will be used."
+				          << std::endl;
+			}
+
+			result = result[0];
+		}
+
+		if (!result.is_object())
+		{
+			std::cerr
+			  << "error: expected results to be either an array or an object."
+			  << std::endl;
+			return result_json;
+		}
+
+		if (!result.contains("expressions"))
+		{
+			std::cerr << "error: result object does not contain 'expressions'"
+			          << std::endl;
+			return result_json;
+		}
+
+		auto &expressions = result["expressions"];
+		if (!expressions.is_array())
+		{
+			std::cerr << "error: expected 'expressions' to be an array"
+			          << std::endl;
+			return result_json;
+		}
+
+		if (expressions.empty())
+		{
+			std::cerr << "warning: query returned no results." << std::endl;
+			return result_json;
+		}
+
+		return expressions[0].dump();
+	}
 } // namespace
 
 int main(int argc, char **argv)
@@ -287,5 +354,6 @@ int main(int argc, char **argv)
 	{
 		rego.add_module_file(modulePath);
 	}
-	std::cout << rego.query(query) << std::endl;
+	std::cout << extract_first_expression_from_result(rego.query(query))
+	          << std::endl;
 }