--- a/sys/dev/misc/nmdm/nmdm.c
+++ b/sys/dev/misc/nmdm/nmdm.c
@@ -562,7 +562,7 @@
 			ourpart->modemsignals |= TIOCM_DTR;
 			break;
 		case TIOCCDTR:
-			ourpart->modemsignals &= TIOCM_DTR;
+			ourpart->modemsignals &= ~TIOCM_DTR;
 			break;
 		case TIOCMSET:
 			ourpart->modemsignals = *(int *)ap->a_data;