diff --git a/lib/r10k/source/exec.rb b/lib/r10k/source/exec.rb
index 0ff4f3bac..954cdd1ee 100644
--- a/lib/r10k/source/exec.rb
+++ b/lib/r10k/source/exec.rb
@@ -28,6 +28,11 @@ def initialize(name, basedir, options = {})
       end
     end
 
+    unless R10K::Source::Hash.valid_environment_hash?(environments)
+      raise R10K::Error, _("Environment source ${name} command %{cmd} did not return valid environment data.\n" \
+                           'Returned: %{dat}') % {name: name, cmd: command, dat: environments}
+    end
+
     # Set the environments key for the parent class to consume
     options[:environments] = environments
 
diff --git a/lib/r10k/source/hash.rb b/lib/r10k/source/hash.rb
index 1e542bd18..2556662bd 100644
--- a/lib/r10k/source/hash.rb
+++ b/lib/r10k/source/hash.rb
@@ -122,6 +122,14 @@ class R10K::Source::Hash < R10K::Source::Base
 
   include R10K::Logging
 
+  # @param hash [Hash] A hash to validate.
+  # @return [Boolean] False if the hash is obviously invalid. A true return
+  #   means _maybe_ it's valid.
+  def self.valid_environment_hash?(hash)
+      # TODO: more robust schema valiation
+      hash.is_a?(Hash)
+  end
+
   # @param name [String] The identifier for this source.
   # @param basedir [String] The base directory where the generated environments will be created.
   # @param options [Hash] An additional set of options for this source. The
diff --git a/spec/unit/source/exec_spec.rb b/spec/unit/source/exec_spec.rb
index d321a3519..2cfc9af05 100644
--- a/spec/unit/source/exec_spec.rb
+++ b/spec/unit/source/exec_spec.rb
@@ -58,7 +58,14 @@
       end
 
       context 'that produces invalid output' do
-        it "rejects schema invalid data"
+        it "rejects strings as schema invalid data" do
+          allow_any_instance_of(R10K::Util::Subprocess)
+            .to receive(:execute)
+            .and_return(double('result', stdout: "200 OK"))
+
+          expect { described_class.new('execsource', '/some/nonexistent/dir', command: '/path/to/command') }
+            .to raise_error(/did not return valid environment data/)
+        end
 
         it 'raises an error for non-json, non-yaml data' do
           allow_any_instance_of(R10K::Util::Subprocess)