diff --git a/Makefile b/Makefile
index b74f74e1..40399c30 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ build-lib:
 	cd src; \
 	  yacc -d parse.y; \
 	  lex scan.l
-	$(CC) $(CFLAGS) -shared src/*.c -o lib/libpicrin.so -I./include -I./extlib
+	$(CC) $(CFLAGS) -shared src/*.c -o lib/libpicrin.so -I./include -I./extlib -lm
 
 clean:
 	rm -f src/y.tab.c src/y.tab.h src/lex.yy.c