diff --git a/configure.ac b/configure.ac
index 09837975418dab69f221caa20f524199129533b7..475449fb99ce2457ca36cda82f9b8486d7ea9296 100644
--- a/configure.ac
+++ b/configure.ac
@@ -283,6 +283,11 @@ else
   PC_BUILD="fixed-point"
   PC_LIBM=
 fi
+dnl opus_custom requires libm as well
+if test x$ac_enable_custom_modes = xyes; then
+  PC_BUILD="${PC_BUILD}, custom modes"
+  PC_LIBM=$LIBM
+fi
 AC_SUBST([PC_BUILD])
 AC_SUBST([PC_LIBM])