diff --git a/internal/config/concurrency_limits_internal_test.go b/internal/config/concurrency_limits_internal_test.go
index c5f4b20..867b041 100644
--- a/internal/config/concurrency_limits_internal_test.go
+++ b/internal/config/concurrency_limits_internal_test.go
@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"runtime"
 	"testing"
 )
@@ -160,3 +161,19 @@ func TestConcurrencyLimitsFromEnvironment(t *testing.T) {
 		})
 	}
 }
+
+// TestInvalidMaxConcurrentProcessingFromEnvironmentAbortsStartup checks
+// that a PIXA_MAX_CONCURRENT_PROCESSING that is not a whole number of at
+// least 1, an empty one included, aborts startup naming the variable and
+// the key instead of taking the default.
+func TestInvalidMaxConcurrentProcessingFromEnvironmentAbortsStartup(t *testing.T) {
+	for _, value := range []string{"", " ", "0.5", "half", "-1", "4 images"} {
+		t.Run(fmt.Sprintf("value %q", value), func(t *testing.T) {
+			t.Setenv(testMaxConcurrentProcessingVar, value)
+
+			_, err := configFromYAML(t, signingKeyLine)
+			wantStartupError(t, err,
+				testMaxConcurrentProcessingVar, keyMaxConcurrentProcessing)
+		})
+	}
+}