diff --git a/environment/pipeline.go b/environment/pipeline.go
index d94507dd..ceea712f 100644
--- a/environment/pipeline.go
+++ b/environment/pipeline.go
@@ -512,6 +512,10 @@ func (p *Pipeline) mount(ctx context.Context, dest string, mnt *compiler.Value)
 	}
 
 	// eg. mount: "/foo": { from: www.source }
+	if !mnt.Lookup("from").Exists() {
+		return nil, fmt.Errorf("invalid mount: should have %s structure",
+			"{from: _, path: string | *\"/\"}")
+	}
 	from := NewPipeline(mnt.Lookup("from"), p.s)
 	if err := from.Run(ctx); err != nil {
 		return nil, err
diff --git a/stdlib/dagger/op/op.cue b/stdlib/dagger/op/op.cue
index 9e492089..44c2f7dd 100644
--- a/stdlib/dagger/op/op.cue
+++ b/stdlib/dagger/op/op.cue
@@ -50,7 +50,13 @@ package op
 	// `true` means also ignoring the mount cache volumes
 	always?: true | *false
 	dir:     string | *"/"
-	mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"} | {secret: _}
+	// HACK: FIXME later [Performance related]
+	// mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"} | {secret: _}
+	// https://github.com/dagger/dagger/issues/856
+	mount: [string]: {
+		_
+		...
+	}
 	// Map of hostnames to ip
 	hosts?: [string]: string
 	// User to exec with (if left empty, will default to the set user in the image)