diff --git a/.gitea/workflows/dotnet-build-test-postgres.yaml b/.gitea/workflows/dotnet-build-test-postgres.yaml
index b3bee4c..5f1faeb 100644
--- a/.gitea/workflows/dotnet-build-test-postgres.yaml
+++ b/.gitea/workflows/dotnet-build-test-postgres.yaml
@@ -18,7 +18,7 @@ on:
         required: false
         default: "Release"
         type: string
-      # string, not boolean: act_runner mangles boolean defaults (format() yields '%!t(string=false)').
+      # string, not boolean: act_runner mangles boolean defaults. The value may still arrive as '%!t(string=…)', see Build step.
       warnaserror:
         description: "Treat build warnings as errors (\"true\"/\"false\")"
         required: false
@@ -101,9 +101,11 @@ jobs:
       - name: Build
         run: |
           ARGS=(-c "$CONFIGURATION" --no-restore)
-          if [ "${WARNASERROR,,}" != "false" ]; then
-            ARGS+=(-warnaserror)
-          fi
+          # act_runner may deliver the value mangled as '%!t(string=false)', so match by substring.
+          case "${WARNASERROR,,}" in
+            *false*) ;;
+            *) ARGS+=(-warnaserror) ;;
+          esac
           set -x
           dotnet build ${SOLUTION:+"$SOLUTION"} "${ARGS[@]}"
 
diff --git a/.gitea/workflows/dotnet-build-test.yaml b/.gitea/workflows/dotnet-build-test.yaml
index ccee438..ef6da49 100644
--- a/.gitea/workflows/dotnet-build-test.yaml
+++ b/.gitea/workflows/dotnet-build-test.yaml
@@ -18,7 +18,7 @@ on:
         required: false
         default: "Release"
         type: string
-      # string, not boolean: act_runner mangles boolean defaults (format() yields '%!t(string=false)').
+      # string, not boolean: act_runner mangles boolean defaults. The value may still arrive as '%!t(string=…)', see Build step.
       warnaserror:
         description: "Treat build warnings as errors (\"true\"/\"false\")"
         required: false
@@ -66,9 +66,11 @@ jobs:
       - name: Build
         run: |
           ARGS=(-c "$CONFIGURATION" --no-restore)
-          if [ "${WARNASERROR,,}" != "false" ]; then
-            ARGS+=(-warnaserror)
-          fi
+          # act_runner may deliver the value mangled as '%!t(string=false)', so match by substring.
+          case "${WARNASERROR,,}" in
+            *false*) ;;
+            *) ARGS+=(-warnaserror) ;;
+          esac
           set -x
           dotnet build ${SOLUTION:+"$SOLUTION"} "${ARGS[@]}"
 
diff --git a/README.md b/README.md
index 313641a..30a8302 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ jobs:
 | `solution` | `""` | Solution/project file, relative to `working-directory`. If empty, dotnet uses the single sln/proj in that directory. |
 | `dotnet-version` | `10.0.x` | .NET SDK version |
 | `configuration` | `Release` | Build configuration |
-| `warnaserror` | `"true"` | Pass `-warnaserror` to `dotnet build` unless the value is `"false"` |
+| `warnaserror` | `"true"` | Pass `-warnaserror` to `dotnet build` unless the value contains `false` |
 | `working-directory` | `.` | Where dotnet commands run |
 | `test-filter` | `""` | Optional `dotnet test --filter` expression |