diff --git a/mysql-test/r/select.result b/mysql-test/r/select.result
index e6c590489a0..7f01d453906 100644
--- a/mysql-test/r/select.result
+++ b/mysql-test/r/select.result
@@ -3395,3 +3395,6 @@ a	t1.b + 0	t1.c + 0	a	t2.b + 0	c	d
 1	0	1	1	0	1	NULL
 2	0	1	NULL	NULL	NULL	NULL
 drop table t1,t2;
+SELECT 0.9888889889 * 1.011111411911;
+0.9888889889 * 1.011111411911
+0.9998769417899202067879
diff --git a/mysql-test/t/select.test b/mysql-test/t/select.test
index b75d0dd8bb6..b44f682c02e 100644
--- a/mysql-test/t/select.test
+++ b/mysql-test/t/select.test
@@ -2901,3 +2901,8 @@ from t1 left outer join t2 on t1.a = t2.c and t2.b <> 1
 where t1.b <> 1 order by t1.a;
 
 drop table t1,t2;
+
+#
+# Bug #20569: Garbage in DECIMAL results from some mathematical functions
+#
+SELECT 0.9888889889 * 1.011111411911;
diff --git a/strings/decimal.c b/strings/decimal.c
index 8786a513945..ecd92a54cc9 100644
--- a/strings/decimal.c
+++ b/strings/decimal.c
@@ -170,6 +170,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
 #define ADD(to, from1, from2, carry)  /* assume carry <= 1 */           \
         do                                                              \
         {                                                               \
+          DBUG_ASSERT((carry) <= 1);                                    \
           dec1 a=(from1)+(from2)+(carry);                               \
           if (((carry)= a >= DIG_BASE)) /* no division here! */         \
             a-=DIG_BASE;                                                \
@@ -179,7 +180,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
 #define ADD2(to, from1, from2, carry)                                   \
         do                                                              \
         {                                                               \
-          dec1 a=(from1)+(from2)+(carry);                               \
+          dec2 a=((dec2)(from1))+(from2)+(carry);                       \
           if (((carry)= a >= DIG_BASE))                                 \
             a-=DIG_BASE;                                                \
           if (unlikely(a >= DIG_BASE))                                  \
@@ -187,7 +188,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
             a-=DIG_BASE;                                                \
             carry++;                                                    \
           }                                                             \
-          (to)=a;                                                       \
+          (to)=(dec1) a;                                                \
         } while(0)
 
 #define SUB(to, from1, from2, carry) /* to=from1-from2 */               \
@@ -1998,7 +1999,13 @@ int decimal_mul(decimal_t *from1, decimal_t *from2, decimal_t *to)
       ADD2(*buf0, *buf0, lo, carry);
       carry+=hi;
     }
-    for (; carry; buf0--)
+    if (carry)
+    {
+      if (buf0 < to->buf)
+        return E_DEC_OVERFLOW;
+      ADD2(*buf0, *buf0, 0, carry);
+    }
+    for (buf0--; carry; buf0--)
     {
       if (buf0 < to->buf)
         return E_DEC_OVERFLOW;