Index: bp-core/bp-core-update.php
===================================================================
--- bp-core/bp-core-update.php
+++ bp-core/bp-core-update.php
@@ -459,10 +459,8 @@
 function bp_core_signups_table_exists() {
 	global $wpdb;
 
-	// Some installations may already have a signups table (multisite, plugins, etc...)
-	if ( ! empty( $wpdb->signups ) ) {
-		return true;
-	}
+	// Never use bp_core_get_table_prefix() for any global users tables
+	$wpdb->signups = $wpdb->base_prefix . 'signups';
 
 	// Suppress errors because users shouldn't see this
 	$old_suppress = $wpdb->suppress_errors();
@@ -492,10 +490,8 @@
 function bp_core_signups_id_column_exists() {
 	global $wpdb;
 
-	// No signups table to query, so bail and return false
-	if ( empty( $wpdb->signups ) ) {
-		return false;
-	}
+	// Never use bp_core_get_table_prefix() for any global users tables
+	$wpdb->signups = $wpdb->base_prefix . 'signups';
 
 	// Suppress errors because users shouldn't see this
 	$old_suppress = $wpdb->suppress_errors();