diff --git a/d2compiler/compile.go b/d2compiler/compile.go
index 8c60316cd..1c83edf99 100644
--- a/d2compiler/compile.go
+++ b/d2compiler/compile.go
@@ -270,6 +270,9 @@ func (c *compiler) compileLabel(attrs *d2graph.Attributes, f d2ir.Node) {
 		// TODO: Delete instead.
 		attrs.Label.Value = scalar.ScalarString()
 	case *d2ast.BlockString:
+		if scalar.ScalarString() == "" {
+			c.errorf(f.LastPrimaryKey(), "block string cannot be empty")
+		}
 		attrs.Language = scalar.Tag
 		fullTag, ok := ShortToFullLanguageAliases[scalar.Tag]
 		if ok {