diff --git a/.github/workflows/linux_unit_tests.yaml b/.github/workflows/linux_unit_tests.yaml
index 47adfb3e8f..c6b9309e99 100644
--- a/.github/workflows/linux_unit_tests.yaml
+++ b/.github/workflows/linux_unit_tests.yaml
@@ -165,3 +165,54 @@ jobs:
       - uses: codecov/codecov-action@v1
         with:
           flags: unittests,linux,clingo
+  clingo-cffi:
+    # Test for the clingo based solver using the CFFI package
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+      - name: Install System packages
+        run: |
+            sudo apt-get -y update
+            # Needed for unit tests
+            sudo apt-get install -y coreutils gfortran graphviz gnupg2 mercurial
+            sudo apt-get install -y ninja-build patchelf
+            # Needed for kcov
+            sudo apt-get -y install cmake binutils-dev libcurl4-openssl-dev
+            sudo apt-get -y install zlib1g-dev libdw-dev libiberty-dev
+      - name: Install Python packages
+        run: |
+            pip install --upgrade pip six setuptools codecov coverage cffi
+            pip install -i https://test.pypi.org/simple/ clingo-cffi
+      - name: Setup git configuration
+        run: |
+            # Need this for the git tests to succeed.
+            git --version
+            . .github/workflows/setup_git.sh
+      - name: Install kcov for bash script coverage
+        env:
+            KCOV_VERSION: 34
+        run: |
+            KCOV_ROOT=$(mktemp -d)
+            wget --output-document=${KCOV_ROOT}/${KCOV_VERSION}.tar.gz https://github.com/SimonKagstrom/kcov/archive/v${KCOV_VERSION}.tar.gz
+            tar -C ${KCOV_ROOT} -xzvf ${KCOV_ROOT}/${KCOV_VERSION}.tar.gz
+            mkdir -p ${KCOV_ROOT}/build
+            cd ${KCOV_ROOT}/build && cmake -Wno-dev ${KCOV_ROOT}/kcov-${KCOV_VERSION} && cd -
+            make -C ${KCOV_ROOT}/build && sudo  make -C ${KCOV_ROOT}/build install
+      - name: Run unit tests
+        run: |
+          whoami && echo PWD=$PWD && echo HOME=$HOME && echo SPACK_TEST_SOLVER=$SPACK_TEST_SOLVER
+          python -c "import clingo; print(hasattr(clingo.Symbol, '_rep'), clingo.__version__)"
+          . share/spack/setup-env.sh
+          spack compiler find
+          spack solve mpileaks%gcc
+          coverage run $(which spack) unit-test -v
+          coverage combine
+          coverage xml
+      - uses: codecov/codecov-action@v1
+        with:
+          flags: unittests,linux,clingo
diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index 3c00f4a968..661a4582ae 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -18,6 +18,8 @@
 
 try:
     import clingo
+    # There may be a better way to detect this
+    clingo_cffi = hasattr(clingo.Symbol, '_rep')
 except ImportError:
     clingo = None  # type: ignore
 
@@ -119,11 +121,11 @@ def __call__(self, *args):
     def symbol(self, positive=True):
         def argify(arg):
             if isinstance(arg, bool):
-                return str(arg)
+                return clingo.String(str(arg))
             elif isinstance(arg, int):
-                return arg
+                return clingo.Number(arg)
             else:
-                return str(arg)
+                return clingo.String(str(arg))
         return clingo.Function(
             self.name, [argify(arg) for arg in self.args], positive=positive)
 
@@ -318,18 +320,26 @@ def solve(
         def on_model(model):
             models.append((model.cost, model.symbols(shown=True, terms=True)))
 
-        solve_result = self.control.solve(
-            assumptions=self.assumptions,
-            on_model=on_model,
-            on_core=cores.append
-        )
+        solve_kwargs = {"assumptions": self.assumptions,
+                        "on_model": on_model,
+                        "on_core": cores.append}
+        if clingo_cffi:
+            solve_kwargs["on_unsat"] = cores.append
+        solve_result = self.control.solve(**solve_kwargs)
         timer.phase("solve")
 
         # once done, construct the solve result
         result.satisfiable = solve_result.satisfiable
 
         def stringify(x):
-            return x.string or str(x)
+            if clingo_cffi:
+                # Clingo w/ CFFI will throw an exception on failure
+                try:
+                    return x.string
+                except RuntimeError:
+                    return str(x)
+            else:
+                return x.string or str(x)
 
         if result.satisfiable:
             builder = SpecBuilder(specs)