diff --git a/lib/fusesocLib.nix b/lib/fusesocLib.nix
index 0fe720f..df62d61 100644
--- a/lib/fusesocLib.nix
+++ b/lib/fusesocLib.nix
@@ -105,6 +105,17 @@ rec {
     in
     parsed // { inherit condition; };
 
+  stripCond =
+    cond:
+    let
+      # extract the argument from conditional syntax
+      # e.g.
+      # "tool_quartus ? (data/fifo.sdc)" -> ["data/fifo.sdc"]
+      # "rtl/fifo.v" -> Null
+      stripped = builtins.match ''^!?[_a-zA-Z]*[[:space:]]*\??[[:space:]]*\((.*)\)$'' cond;
+    in
+    if (isNull stripped) then cond else builtins.head stripped;
+
   resolveDeps = import ./resolveDeps.nix { inherit lib parseDependency; };
 
   mkCoreSet =
@@ -215,12 +226,13 @@ rec {
 
   importCore = import ./importCore.nix {
     inherit
-      lib
       formats
-      readYAML
-      stdenvNoCC
+      lib
       mkRunners
       parseVlnv
+      readYAML
+      stdenvNoCC
+      stripCond
       ;
   };
 
diff --git a/lib/importCore.nix b/lib/importCore.nix
index 6d043d0..d399668 100644
--- a/lib/importCore.nix
+++ b/lib/importCore.nix
@@ -1,10 +1,11 @@
 {
-  lib,
   formats,
-  readYAML,
-  stdenvNoCC,
+  lib,
   mkRunners,
   parseVlnv,
+  readYAML,
+  stdenvNoCC,
+  stripCond,
 }:
 lib.extendMkDerivation {
   constructDrv = stdenvNoCC.mkDerivation;
@@ -25,13 +26,15 @@ lib.extendMkDerivation {
     }:
     let
       useCoreFile = isNull core;
-      core' = if useCoreFile then readYAML "${coreRoot}/${coreName}" else core;
+      core' = if useCoreFile then (readYAML "${coreRoot}/${coreName}") else core;
       parsed = parseVlnv finalAttrs.passthru.core.name;
+
       coreFileset = builtins.concatLists (
         lib.mapAttrsToList (
           n: v:
           map (
-            relPath: if builtins.isAttrs relPath then (builtins.head (builtins.attrNames relPath)) else relPath
+            entry:
+            stripCond (if builtins.isAttrs entry then (builtins.head (builtins.attrNames entry)) else entry)
           ) (v.files or [ ])
         ) finalAttrs.passthru.core.filesets
       );