diff --git a/src/core/dhcp/nm-dhcp-client.c b/src/core/dhcp/nm-dhcp-client.c
index 0db931ddcd..3b0343fdd7 100644
--- a/src/core/dhcp/nm-dhcp-client.c
+++ b/src/core/dhcp/nm-dhcp-client.c
@@ -84,6 +84,11 @@ G_DEFINE_ABSTRACT_TYPE(NMDhcpClient, nm_dhcp_client, G_TYPE_OBJECT)
 
 /*****************************************************************************/
 
+/* we use pid=-1 for invalid PIDs. Ensure that pid_t can hold negative values. */
+G_STATIC_ASSERT(!(((pid_t) -1) > 0));
+
+/*****************************************************************************/
+
 pid_t
 nm_dhcp_client_get_pid(NMDhcpClient *self)
 {