diff --git a/files.go b/files.go
index ce3cdfd12..a92a14091 100644
--- a/files.go
+++ b/files.go
@@ -95,7 +95,7 @@ func (s *SourceInlineDir) validate() error {
 
 	for k, d := range s.Dirs {
 		if strings.ContainsRune(k, os.PathSeparator) {
-			errs = append(errs, errors.Errorf("dir name %q must not contain path separator", k))
+			errs = append(errs, errors.Wrapf(sourceNamePathSeparatorError, "dir %q", k))
 		}
 
 		if err := d.validate(); err != nil {
@@ -105,7 +105,7 @@ func (s *SourceInlineDir) validate() error {
 
 	for k, f := range s.Files {
 		if strings.ContainsRune(k, os.PathSeparator) {
-			errs = append(errs, errors.Errorf("file name %q must not contain path separator", k))
+			errs = append(errs, errors.Wrapf(sourceNamePathSeparatorError, "file %q", k))
 		}
 		if err := f.validate(); err != nil {
 			errs = append(errs, errors.Wrapf(err, "file %q", k))
diff --git a/load.go b/load.go
index 07572df6b..f74257590 100644
--- a/load.go
+++ b/load.go
@@ -3,6 +3,7 @@ package dalec
 import (
 	goerrors "errors"
 	"fmt"
+	"os"
 	"path"
 	"strings"
 
@@ -370,14 +371,17 @@ func (s *Spec) FillDefaults() {
 
 func (s Spec) Validate() error {
 	for name, src := range s.Sources {
+		if strings.ContainsRune(name, os.PathSeparator) {
+			return &InvalidSourceError{Name: name, Err: sourceNamePathSeparatorError}
+		}
 		if err := src.validate(); err != nil {
-			return fmt.Errorf("error validating source ref %q: %w", name, err)
+			return &InvalidSourceError{Name: name, Err: fmt.Errorf("error validating source ref %q: %w", name, err)}
 		}
 
 		if src.DockerImage != nil && src.DockerImage.Cmd != nil {
 			for p, cfg := range src.DockerImage.Cmd.CacheDirs {
 				if _, err := sharingMode(cfg.Mode); err != nil {
-					return errors.Wrapf(err, "invalid sharing mode for source %q with cache mount at path %q", name, p)
+					return &InvalidSourceError{Name: name, Err: errors.Wrapf(err, "invalid sharing mode for source %q with cache mount at path %q", name, p)}
 				}
 			}
 		}
diff --git a/load_test.go b/load_test.go
index 97ed240a6..d4334b4d3 100644
--- a/load_test.go
+++ b/load_test.go
@@ -2,6 +2,7 @@ package dalec
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -443,3 +444,33 @@ sources:
 		})
 	}
 }
+
+func TestSourceNameWithPathSeparator(t *testing.T) {
+	spec := &Spec{
+		Sources: map[string]Source{
+			"forbidden/name": {
+				Inline: &SourceInline{
+					File: &SourceInlineFile{},
+				},
+			},
+		},
+	}
+
+	err := spec.Validate()
+	if err == nil {
+		t.Fatal("expected error, but received none")
+	}
+
+	var expected *InvalidSourceError
+	if !errors.As(err, &expected) {
+		t.Fatalf("expected %T, got %T", expected, err)
+	}
+
+	if expected.Name != "forbidden/name" {
+		t.Error("expected error to contain source name")
+	}
+
+	if !errors.Is(err, sourceNamePathSeparatorError) {
+		t.Errorf("expected error to be sourceNamePathSeparatorError, got: %v", err)
+	}
+}
diff --git a/source.go b/source.go
index dc38fdcf7..b8d3d4fe2 100644
--- a/source.go
+++ b/source.go
@@ -11,8 +11,25 @@ import (
 	"github.com/moby/buildkit/frontend/dockerui"
 	"github.com/moby/buildkit/identity"
 	"github.com/moby/buildkit/util/gitutil"
+	"github.com/pkg/errors"
 )
 
+// InvalidSourceError is an error type returned when a source is invalid.
+type InvalidSourceError struct {
+	Name string
+	Err  error
+}
+
+func (s *InvalidSourceError) Error() string {
+	return fmt.Sprintf("invalid source %s: %v", s.Name, s.Err)
+}
+
+func (s *InvalidSourceError) Unwrap() error {
+	return s.Err
+}
+
+var sourceNamePathSeparatorError = errors.New("source name must not container path separator")
+
 type LLBGetter func(sOpts SourceOpts, opts ...llb.ConstraintsOpt) (llb.State, error)
 
 type ForwarderFunc func(llb.State, *SourceBuild) (llb.State, error)