diff --git a/psutil/_psutil_linux.c b/psutil/_psutil_linux.c
index 1ed1f91a0..0bacb2048 100644
--- a/psutil/_psutil_linux.c
+++ b/psutil/_psutil_linux.c
@@ -58,6 +58,11 @@ static const int NCPUS_START = sizeof(unsigned long) * CHAR_BIT;
 #endif
 
 
+#ifndef SPEED_UNKNOWN
+    #define SPEED_UNKNOWN -1
+#endif
+
+
 #if PSUTIL_HAVE_IOPRIO
 enum {
     IOPRIO_WHO_PROCESS = 1,