diff --git a/Makefile b/Makefile
index 4f392bc..0a9f6b5 100644
--- a/Makefile
+++ b/Makefile
@@ -1,8 +1,10 @@
 APPNAME ?= jpegqs
 MPFLAGS ?= -fopenmp
-CFLAGS ?= -Wall -O2 $(MPFLAGS) -DAPPNAME=$(APPNAME)
-LDFLAGS ?= -ljpeg -lm
+CFLAGS ?= -Wall -O3 $(MPFLAGS) -DAPPNAME=$(APPNAME)
+LDFLAGS ?= -ljpeg -lm -s
 SRCS = src
+OBJS = $(SRCS)/idct.o $(SRCS)/libjpegqs.o
+OBJB = $(SRCS)/jpegqs.o
 
 ifneq ($(shell uname -m), i386)
     CFLAGS += -fPIC
@@ -16,16 +18,16 @@ PLIBS = lib$(APPNAME).a lib$(APPNAME).so.0
 all: $(PLIBS) $(PROGS)
 
 clean:
-	rm -rf $(PLIBS) $(PROGS) $(SRCS)/*.o
+	rm -rf $(PLIBS) $(PROGS) $(OBJS) $(OBJB)
 
-lib$(APPNAME).a: $(SRCS)/idct.o $(SRCS)/libjpegqs.o
+lib$(APPNAME).a: $(OBJS)
 	$(AR) rcs $@ $^
 
-lib$(APPNAME).so.0: $(SRCS)/idct.o $(SRCS)/libjpegqs.o
-	$(CC) $(CFLAGS) -shared $^ -o $@ $(LDFLAGS) -s
+lib$(APPNAME).so.0: $(OBJS)
+	$(CC) $(CFLAGS) -shared $^ -o $@ $(LDFLAGS)
 
-$(APPNAME): $(SRCS)/jpegqs.o lib$(APPNAME).so.0
-	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS) -s
+$(APPNAME): $(OBJB) lib$(APPNAME).so.0
+	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS)
 
-$(APPNAME)-static: $(SRCS)/jpegqs.c lib$(APPNAME).a
-	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS) -s
+$(APPNAME)-static: $(OBJB) lib$(APPNAME).a
+	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS)
diff --git a/src/libjpegqs.c b/src/libjpegqs.c
index 8ef698c..5ce53fb 100644
--- a/src/libjpegqs.c
+++ b/src/libjpegqs.c
@@ -302,6 +302,32 @@ void quantsmooth_transform(j_decompress_ptr srcinfo, jvirt_barray_ptr *src_coef_
                 }
             }
         } // iter
+
+        // fix a rare JERR_BAD_DCT_COEF ("DCT coefficient out of range") error
+        // it doesn't take much time
+#ifdef _OPENMP
+#  pragma omp parallel for schedule(dynamic)
+#endif
+        for (blk_y = 0; blk_y < comp_height; blk_y++)
+        {
+            JDIMENSION blk_x;
+            JBLOCKARRAY buffer = (*srcinfo->mem->access_virt_barray)
+                    ((j_common_ptr)srcinfo, src_coef_arrays[ci], blk_y, 1, TRUE);
+
+            for (blk_x = 0; blk_x < comp_width; blk_x++)
+            {
+                JCOEFPTR coef = buffer[0][blk_x]; int i;
+                for (i = 0; i < DCTSIZE2; i++)
+                {
+                    // MAX_COEF_BITS = BITS_IN_JSAMPLE + 2
+                    int lim = (4 << BITS_IN_JSAMPLE) - 1;
+                    int a = coef[i];
+                    a = (a < -lim) ? -lim : (a < lim) ? a : lim;
+                    coef[i] = a;
+                }
+            }
+        }
+
 #undef IMAGEPTR
         free(image - 7);
     }