diff --git a/src/Prometheus.jl b/src/Prometheus.jl
index 5813b0a..8a38c5b 100644
--- a/src/Prometheus.jl
+++ b/src/Prometheus.jl
@@ -631,6 +631,6 @@ include("process_collector.jl")
 # Default registry and collectors
 const DEFAULT_REGISTRY = CollectorRegistry()
 const GC_COLLECTOR = GCCollector(; registry=DEFAULT_REGISTRY)
-const PROCESS_COLLECTOR = ProcessCollector(DEFAULT_REGISTRY)
+const PROCESS_COLLECTOR = ProcessCollector(; registry=DEFAULT_REGISTRY)
 
 end # module Prometheus
diff --git a/src/process_collector.jl b/src/process_collector.jl
index 8d77456..74cdc93 100644
--- a/src/process_collector.jl
+++ b/src/process_collector.jl
@@ -3,12 +3,13 @@
 #################################
 
 mutable struct ProcessCollector <: Collector
-    @const pid_f::Function
+    @const pid::Function
     @const system_boot_time::Int
     @const clock_ticks_per_second::Int
     @const pagesize::Int
     function ProcessCollector(
-            registry::Union{CollectorRegistry, Nothing}, pid_f::Function = () -> "self",
+            pid::Function = () -> "self";
+            registry::Union{CollectorRegistry, Nothing}=DEFAULT_REGISTRY,
         )
         # Read boot time as a way to check if /proc is available and readable
         system_boot_time = 0
@@ -44,14 +45,13 @@ mutable struct ProcessCollector <: Collector
             end
         end
         # Create the collector
-        procc = new(pid_f, system_boot_time, clock_ticks_per_second, pagesize)
+        procc = new(pid, system_boot_time, clock_ticks_per_second, pagesize)
         if registry !== nothing
             register(registry, procc)
         end
         return procc
     end
 end
-ProcessCollector(pid_f::Function = () -> "self") = ProcessCollector(DEFAULT_REGISTRY, pid_f)
 
 function metric_names(::ProcessCollector)
     return (
@@ -68,7 +68,7 @@ function collect!(metrics::Vector, procc::ProcessCollector)
     procc.system_boot_time == 0 && return metrics
     # Fetch the pid
     pid = try
-        strip(string(procc.pid_f()))
+        String(strip(string(procc.pid()::Union{AbstractString,Integer})))::String
     catch e
         @error "ProcessCollector: could not look up the pid from the lambda" e
         return metrics
diff --git a/test/runtests.jl b/test/runtests.jl
index 566cfa9..3a95c1a 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -318,7 +318,7 @@ end
 
 @testset "Prometheus.ProcessCollector" begin
     r = Prometheus.CollectorRegistry()
-    c = Prometheus.ProcessCollector(r)
+    c = Prometheus.ProcessCollector(; registry=r)
     @test c in r.collectors
     metrics = Prometheus.collect(c)
     procfs_available = c.system_boot_time > 0
@@ -368,7 +368,7 @@ end
         @test isempty(metrics)
     end
     # Test that pid function works
-    procc = Prometheus.ProcessCollector(nothing, () -> getpid())
+    procc = Prometheus.ProcessCollector(() -> getpid(); registry=nothing)
     metrics = Prometheus.collect(procc)
     if procfs_available
         @test length(metrics) > 0