diff --git a/Zend/Optimizer/dce.c b/Zend/Optimizer/dce.c
index 3fec874a722..1af15452036 100644
--- a/Zend/Optimizer/dce.c
+++ b/Zend/Optimizer/dce.c
@@ -464,13 +464,19 @@ static inline int get_common_phi_source(zend_ssa *ssa, zend_ssa_phi *phi) {
 	int common_source = -1;
 	int source;
 	FOREACH_PHI_SOURCE(phi, source) {
+		if (source == phi->ssa_var) {
+			continue;
+		}
 		if (common_source == -1) {
 			common_source = source;
-		} else if (common_source != source && source != phi->ssa_var) {
+		} else if (common_source != source) {
 			return -1;
 		}
 	} FOREACH_PHI_SOURCE_END();
-	ZEND_ASSERT(common_source != -1);
+
+	/* If all sources are phi->ssa_var this phi must be in an unreachable cycle.
+	 * We can't easily drop the phi in that case, as we don't have something to replace it with.
+	 * Ideally SCCP would eliminate the whole cycle. */
 	return common_source;
 }
 
diff --git a/Zend/tests/unreachable_phi_cycle.phpt b/Zend/tests/unreachable_phi_cycle.phpt
new file mode 100644
index 00000000000..84bfec214bb
--- /dev/null
+++ b/Zend/tests/unreachable_phi_cycle.phpt
@@ -0,0 +1,15 @@
+--TEST--
+Unreachable phi cycle
+--FILE--
+<?php
+// The inner loop is dead, but SCCP reachability analysis doesn't realize this,
+// as this is determined based on type information.
+function test() {
+    for (; $i--;) {
+        for(; x;);
+    }
+}
+test();
+?>
+--EXPECTF--
+Warning: Undefined variable $i in %s on line %d