diff --git a/internal/source/source.go b/internal/source/source.go
index 4841877..28784d4 100644
--- a/internal/source/source.go
+++ b/internal/source/source.go
@@ -46,42 +46,59 @@ type Source struct {
 func Open(domain, flowID string) (*Source, error) {
 	inst, err := mxl.NewInstance(domain, "")
 	if err != nil {
-		return nil, fmt.Errorf("NewInstance: %w", err)
+		return nil, wrapError("new MXL instance", ErrorKindUnavailable, err)
 	}
 	r, err := inst.NewReader(flowID)
 	if err != nil {
 		inst.Close()
-		return nil, fmt.Errorf("NewReader: %w", err)
+		return nil, wrapError("open video reader", ErrorKindUnavailable, err)
 	}
 	info, err := r.Info()
 	if err != nil {
 		r.Close()
 		inst.Close()
-		return nil, fmt.Errorf("Info: %w", err)
+		return nil, wrapError("get video info", ErrorKindUnavailable, err)
 	}
 	def, err := inst.FlowDef(flowID)
 	if err != nil {
 		r.Close()
 		inst.Close()
-		return nil, fmt.Errorf("FlowDef: %w", err)
+		return nil, wrapError("read video flow definition", ErrorKindUnavailable, err)
 	}
 	var fd flowDef
 	if err := json.Unmarshal([]byte(def), &fd); err != nil {
 		r.Close()
 		inst.Close()
-		return nil, fmt.Errorf("parse flow def: %w", err)
+		return nil, wrapError("parse flow definition JSON", ErrorKindInvalidConfig, err)
 	}
 	if fd.FrameWidth == 0 || fd.FrameHeight == 0 {
 		r.Close()
 		inst.Close()
-		return nil, fmt.Errorf("flow has no video dimensions (not a video flow?)")
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			errors.New("flow has no video dimensions"),
+		)
 	}
 	rate := info.Config.Common.GrainRate
 	idx := mxl.CurrentIndex(rate)
 	if idx == mxl.UndefinedIndex {
 		r.Close()
 		inst.Close()
-		return nil, fmt.Errorf("invalid grain rate: %d/%d", rate.Num, rate.Den)
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			fmt.Errorf("invalid grain rate: %d/%d", rate.Num, rate.Den),
+		)
+	}
+	if len(info.Config.Discrete.SliceSizes) == 0 {
+		r.Close()
+		inst.Close()
+		return nil, wrapError(
+			"validate video flow",
+			ErrorKindInvalidConfig,
+			errors.New("video flow has no slice sizes"),
+		)
 	}
 	return &Source{
 		inst:   inst,
diff --git a/mxl-player b/mxl-player
index 90c2687..6ebe0ba 100755
Binary files a/mxl-player and b/mxl-player differ