diff --git a/src/ops/dbcsr_operations.F b/src/ops/dbcsr_operations.F
index 44112b8cb23..ba624bf6696 100644
--- a/src/ops/dbcsr_operations.F
+++ b/src/ops/dbcsr_operations.F
@@ -1479,6 +1479,9 @@ SUBROUTINE dbcsr_copy_into_existing_sym(matrix_b, matrix_a)
          IF (.NOT. found) THEN
             CALL dbcsr_data_clear(dst_data)
          ELSE
+            IF (dbcsr_data_get_size(src_data) .NE. dbcsr_data_get_size(dst_data)) THEN
+               DBCSR_ABORT("Block sizes not equal!")
+            END IF
             IF (checker_tr(dst_row, dst_col)) THEN
                src_tr = .NOT. src_tr
                CALL swap(src_rs, src_cs)