diff --git a/sfizz/OnePoleFilter.h b/sfizz/OnePoleFilter.h
index 6c147199..14d080ac 100644
--- a/sfizz/OnePoleFilter.h
+++ b/sfizz/OnePoleFilter.h
@@ -56,7 +56,7 @@ public:
         auto in = input.begin();
         auto out = lowpass.begin();
         auto size = std::min(input.size(), lowpass.size());
-        auto sentinel = in.begin() + size;
+        auto sentinel = in + size;
         while (in < sentinel) {
             oneLowpass(in, out);
             in++;
@@ -70,7 +70,7 @@ public:
         auto in = input.begin();
         auto out = highpass.begin();
         auto size = std::min(input.size(), highpass.size());
-        auto sentinel = in.begin() + size;
+        auto sentinel = in + size;
         while (in < sentinel) {
             oneHighpass(in, out);
             in++;
@@ -85,7 +85,7 @@ public:
         auto out = lowpass.begin();
         auto g = gain.begin();
         auto size = min(input.size(), lowpass.size(), gain.size());
-        auto sentinel = in.begin() + size;
+        auto sentinel = in + size;
         while (in < sentinel) {
             setGain(*g);
             oneLowpass(in, out);
@@ -99,10 +99,10 @@ public:
         auto out = highpass.begin();
         auto g = gain.begin();
         auto size = min(input.size(), highpass.size(), gain.size());
-        auto sentinel = in.begin() + size;
+        auto sentinel = in + size;
         while (in < sentinel) {
             setGain(*g);
-            oneHighpass(in, out);
+            oneLowpass(in, out);
         }
         return size;
     }
diff --git a/tests/OnePoleFilterT.cpp b/tests/OnePoleFilterT.cpp
index 56fe281c..14cadcaf 100644
--- a/tests/OnePoleFilterT.cpp
+++ b/tests/OnePoleFilterT.cpp
@@ -24,9 +24,9 @@
 #include "OnePoleFilter.h"
 #include "catch2/catch.hpp"
 #include "cnpy.h"
+#include "../sfizz/compat/filesystem.h"
 #include <absl/types/span.h>
 #include <algorithm>
-#include <filesystem>
 #include <string>
 using namespace Catch::literals;
 
@@ -67,7 +67,7 @@ void testLowpass(const std::filesystem::path& inputNumpyFile, const std::filesys
     for (auto& data : outputSpan)
         expectedData.push_back(static_cast<Type>(data));
 
-    OnePoleFilter filter { gain };
+    OnePoleFilter<Type> filter { gain };
     std::vector<Type> outputData(size);
     filter.processLowpass(inputData, absl::MakeSpan(outputData));
     REQUIRE(approxEqual(outputData, expectedData));
@@ -102,7 +102,7 @@ void testHighpass(const std::filesystem::path& inputNumpyFile, const std::filesy
     for (auto& data : outputSpan)
         expectedData.push_back(static_cast<Type>(data));
 
-    OnePoleFilter filter { gain };
+    OnePoleFilter<Type> filter { gain };
     std::vector<Type> outputData(size);
     filter.processHighpass(inputData, absl::MakeSpan(outputData));
     REQUIRE(approxEqual(outputData, expectedData));