diff --git a/src/pkg/runtime/malloc.goc b/src/pkg/runtime/malloc.goc
index 0a0420d415..280a0a2a8f 100644
--- a/src/pkg/runtime/malloc.goc
+++ b/src/pkg/runtime/malloc.goc
@@ -224,6 +224,8 @@ largealloc(uint32 flag, uintptr *sizep)
 
 	// Allocate directly from heap.
 	size = *sizep;
+	if(size + PageSize < size)
+		runtime·throw("out of memory");
 	npages = size >> PageShift;
 	if((size & PageMask) != 0)
 		npages++;
diff --git a/src/pkg/runtime/string.goc b/src/pkg/runtime/string.goc
index 407188cfe6..a46fa5d8d2 100644
--- a/src/pkg/runtime/string.goc
+++ b/src/pkg/runtime/string.goc
@@ -334,6 +334,8 @@ func stringtoslicerune(s String) (b Slice) {
 		n++;
 	}
 
+	if(n > MaxMem/sizeof(r[0]))
+		runtime·throw("out of memory");
 	mem = runtime·roundupsize(n*sizeof(r[0]));
 	b.array = runtime·mallocgc(mem, 0, FlagNoScan|FlagNoZero);
 	b.len = n;