diff --git a/mysql-test/main/func_time.result b/mysql-test/main/func_time.result
index 8bd3774162b..cf6d48cdf97 100644
--- a/mysql-test/main/func_time.result
+++ b/mysql-test/main/func_time.result
@@ -6295,3 +6295,12 @@ Warning	1292	Truncated incorrect time value: '50326 garbage'
 SELECT TIME('- 01:00:00'), TIME('- 1 01:00:00');
 TIME('- 01:00:00')	TIME('- 1 01:00:00')
 -01:00:00	-25:00:00
+#
+# MDEV-17854 Assertion `decimals <= 6' failed in my_time_fraction_remainder on SELECT with NULLIF and FROM_UNIXTIME on incorrect time
+#
+SELECT NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14');
+NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14')
+1970-01-01 04:00:00
+Warnings:
+Warning	1292	Truncated incorrect DECIMAL value: 'foo'
+Warning	1292	Truncated incorrect DECIMAL value: 'foo'
diff --git a/mysql-test/main/func_time.test b/mysql-test/main/func_time.test
index e47667ff7a2..e9c83156920 100644
--- a/mysql-test/main/func_time.test
+++ b/mysql-test/main/func_time.test
@@ -3123,3 +3123,10 @@ SELECT
   CAST('50326 garbage' as time);
 
 SELECT TIME('- 01:00:00'), TIME('- 1 01:00:00');
+
+
+--echo #
+--echo # MDEV-17854 Assertion `decimals <= 6' failed in my_time_fraction_remainder on SELECT with NULLIF and FROM_UNIXTIME on incorrect time
+--echo #
+
+SELECT NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14');
diff --git a/sql/sql_type.cc b/sql/sql_type.cc
index cd993e0524e..f92ce45bc97 100644
--- a/sql/sql_type.cc
+++ b/sql/sql_type.cc
@@ -862,7 +862,9 @@ Datetime_truncation_not_needed::Datetime_truncation_not_needed(THD *thd, Item *i
     existed (but we know there were no nanoseconds). Here we assert that there
     are also no microsecond digits outside of the scale specified in "dec".
   */
-  DBUG_ASSERT(!is_valid_datetime() || fraction_remainder(item->decimals) == 0);
+  DBUG_ASSERT(!is_valid_datetime() ||
+              fraction_remainder(MY_MIN(item->decimals,
+                                        TIME_SECOND_PART_DIGITS)) == 0);
 }
 
 /********************************************************************/