diff --git a/src/sfizz/NumericId.h b/src/sfizz/NumericId.h
index b2fed501..5abd906f 100644
--- a/src/sfizz/NumericId.h
+++ b/src/sfizz/NumericId.h
@@ -18,24 +18,35 @@ struct NumericId {
     constexpr NumericId() = default;
 
     explicit constexpr NumericId(int number)
-        : number(number)
+        : number_(number)
     {
     }
 
     constexpr bool valid() const noexcept
     {
-        return number != -1;
+        return number_ != -1;
+    }
+
+    constexpr int number() const noexcept
+    {
+        return number_;
+    }
+
+    explicit operator bool() const noexcept
+    {
+        return valid();
     }
 
     constexpr bool operator==(NumericId other) const noexcept
     {
-        return number == other.number;
+        return number_ == other.number_;
     }
 
     constexpr bool operator!=(NumericId other) const noexcept
     {
-        return number != other.number;
+        return number_ != other.number_;
     }
 
-    const int number = -1;
+private:
+    int number_ = -1;
 };
diff --git a/src/sfizz/Synth.cpp b/src/sfizz/Synth.cpp
index 9a62067a..58669f1d 100644
--- a/src/sfizz/Synth.cpp
+++ b/src/sfizz/Synth.cpp
@@ -1188,10 +1188,10 @@ const sfz::Region* sfz::Synth::getRegionById(NumericId<Region> id) const noexcep
         return nullptr;
 
     // search a sequence of ordered identifiers with potential gaps
-    size_t index = static_cast<size_t>(id.number);
+    size_t index = static_cast<size_t>(id.number());
     index = std::min(index, size - 1);
 
-    while (index > 0 && regions[index]->getId().number > id.number)
+    while (index > 0 && regions[index]->getId().number() > id.number())
         --index;
 
     return (regions[index]->getId() == id) ? regions[index].get() : nullptr;
@@ -1205,10 +1205,10 @@ const sfz::Voice* sfz::Synth::getVoiceById(NumericId<Voice> id) const noexcept
         return nullptr;
 
     // search a sequence of ordered identifiers with potential gaps
-    size_t index = static_cast<size_t>(id.number);
+    size_t index = static_cast<size_t>(id.number());
     index = std::min(index, size - 1);
 
-    while (index > 0 && voices[index]->getId().number > id.number)
+    while (index > 0 && voices[index]->getId().number() > id.number())
         --index;
 
     return (voices[index]->getId() == id) ? voices[index].get() : nullptr;