diff --git a/effekt/jvm/src/test/scala/effekt/StdlibTests.scala b/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
index 58272dbe9..6017edc6d 100644
--- a/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
+++ b/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
@@ -66,9 +66,5 @@ class StdlibLLVMTests extends StdlibTests {
   override def ignored: Set[File] = Set(
     // String comparison using `<`, `<=`, `>`, `>=` is not implemented yet on LLVM
     examplesDir / "stdlib" / "string" / "compare.effekt",
-
-    // Wrong codegen for negative types, see #801
-    examplesDir / "stdlib" / "json.effekt",
-    examplesDir / "stdlib" / "buffer.effekt",
   )
 }
diff --git a/libraries/common/buffer.effekt b/libraries/common/buffer.effekt
index 94d3c18bd..78548d74f 100644
--- a/libraries/common/buffer.effekt
+++ b/libraries/common/buffer.effekt
@@ -48,7 +48,7 @@ def arrayBuffer[T](initialCapacity: Int): Buffer[T] at global = {
     def full?() = capacity() <= 0
     def empty?() = size() <= 0
     def read() = {
-      if (buffer.empty?) None()
+      if (size() <= 0) None()
       else {
         val result: T = contents.unsafeGet(head.get)
         head.set(mod(head.get + 1, initialCapacity))
@@ -56,7 +56,7 @@ def arrayBuffer[T](initialCapacity: Int): Buffer[T] at global = {
       }
     }
     def write(el: T) = {
-      if (buffer.full?) <> // raise(BufferOverflow())
+      if (capacity() <= 0) <> // raise(BufferOverflow())
 
       contents.unsafeSet(tail.get, el)
       tail.set(mod(tail.get + 1, initialCapacity))
diff --git a/libraries/common/json.effekt b/libraries/common/json.effekt
index f6bb773f3..b3db5cc4e 100644
--- a/libraries/common/json.effekt
+++ b/libraries/common/json.effekt
@@ -327,15 +327,18 @@ namespace test {
       def k = new JsonObjectBuilder {
         def field(k){v} = { println(k); ignore{v} }
       }
-      def d = new JsonBuilder {
-        def number(n) = println(n)
-        def bool(b) = ()
-        def null() = println("NULL")
-        def string(s) = ()
-        def list(){e} = handleJsonBuilder{d}{e}
-        def dict(){e} = handleJsonObjectBuilder{k}{e}
+      def jsonBuilder[R] {e: => R / JsonBuilder}: R = {
+        def d = new JsonBuilder {
+          def number(n) = println(n)
+          def bool(b) = ()
+          def null() = println("NULL")
+          def string(s) = ()
+          def list(){e} = jsonBuilder{e}
+          def dict(){e} = handleJsonObjectBuilder{k}{e}
+        }
+        handleJsonBuilder{d}{e}
       }
-      handleJsonBuilder{d}{ decodeJson() }
+      jsonBuilder { decodeJson() }
     }
 
     println("")