diff --git a/common/acpi.c b/common/acpi.c
index 2897ed1f..9dfcd2b6 100644
--- a/common/acpi.c
+++ b/common/acpi.c
@@ -706,11 +706,12 @@ static ACPI_STATUS init_fadt(void) {
 static ACPI_STATUS init_madt(unsigned bsp_cpu_id) {
     ACPI_TABLE_MADT *madt = acpi_find_table(ACPI_SIG_MADT);
     ACPI_SUBTABLE_HEADER *subtbl = (void *) madt + sizeof(*madt);
-    uint32_t length = madt->Header.Length - sizeof(*madt);
 
     if (!madt || !subtbl)
         return AE_ERROR;
 
+    uint32_t length = madt->Header.Length - sizeof(*madt);
+
     acpi_walk_subtables(subtbl, length, madt_parser, (void *) _ul(bsp_cpu_id));
     return AE_OK;
 }