diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 460eea5eaf6..1b823ff4673 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -218,11 +218,14 @@ static Flake getFlake(
 
         if (outputs->value->lambda.fun->matchAttrs) {
             for (auto & formal : outputs->value->lambda.fun->formals->formals) {
-                if (formal.name != state.sSelf)
+                if (formal.name != state.sSelf) {
+                    if (!flake.inputs.count(formal.name))
+                        warn("implicit flake:%s input via output function argument in %s", formal.name, lockedRef);
                     flake.inputs.emplace(formal.name, FlakeInput {
                         .ref = parseFlakeRef(formal.name)
                     });
-            }
+                };
+            };
         }
 
     } else
diff --git a/tests/flakes.sh b/tests/flakes.sh
index 5aec563ac09..014f41c02b5 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -245,6 +245,53 @@ nix build -o $TEST_ROOT/result --no-registries git+file://$flake2Dir#bar --refre
 nix build -o $TEST_ROOT/result $flake3Dir#xyzzy
 git -C $flake3Dir add flake.lock
 
+# Check how non-existent implicit input references are handled with
+# body eval errors
+rm $flake3Dir/flake.nix
+
+cat > $flake3Dir/flake.nix <<EOF
+{
+  description = "Fnord";
+
+  outputs = { self, noflake }: rec {
+    packages.$system.xyzzy = nofluke.packages.$system.foo;
+  };
+}
+EOF
+
+git -C $flake3Dir add flake.nix
+git -C $flake3Dir commit -m 'Update flake.nix with fluke'
+
+err=$flake3Dir/impl.errout
+! nix build -o $TEST_ROOT/result $flake3Dir#xyzzy 2>$err
+cat $err
+grep -q UndefinedVarError $err
+grep -q nofluke $err
+
+# Check how non-existent implicit input references are handled
+rm $flake3Dir/flake.nix
+
+cat > $flake3Dir/flake.nix <<EOF
+{
+  description = "Fnord";
+
+  outputs = { self, noflake }: rec {
+    packages.$system.xyzzy = noflake.packages.$system.foo;
+  };
+}
+EOF
+
+git -C $flake3Dir add flake.nix
+git -C $flake3Dir commit -m 'Update flake.nix with noflake'
+
+err=$flake3Dir/impl.errout
+! nix build -o $TEST_ROOT/result $flake3Dir#xyzzy
+! nix build -o $TEST_ROOT/result $flake3Dir#xyzzy 2>$err
+cat $err
+grep "warning: implicit flake:noflake input via output function argument in git+file:///" $err
+grep error $err
+grep "cannot find flake 'flake:noflake' in the flake registries" $err
+
 # Add dependency to flake3.
 rm $flake3Dir/flake.nix